Geology of the Chewelah-Loon Lake Area, Stevens and Spokane Counties, Washington

GEOLOGICAL SURVEY PROFESSIONAL PAPER 806 Prepared in cooperation with the Washington Division of Mines and Geology

_J Geology of the Chewelah-Loon Lake Area, Stevens and Spokane Counties, Washington ·

By FRED K. MILLER and LORIN D. CLARK

With a section on POTASSIUM-ARGO:N AGES OF THE PLUTONIC ROCKS

By JOAN C. ENGELS

G E 0 L 0 G I CAL SURVEY P.R 0 FE S S I 0 N A. L PAPER 8 0 6

Prepared in cooperation with the Washington Division of Mines and Geology

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1975 UNITED STATES DEPARTMENT OF THE INTERIOR

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY

V. E. McKelvey, Director

/ Library of Congress catalog-card No. 73-600314

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 -Price $2.15 (paper cover) Stock Number 2401-02587 CONTENTS

Page Page Abstract ····································································-·················· 1 Paleozoic rocks-Continued Introduction ...... 1 Carbonate rocks-Continued Location and .accessibility...... -1 -·Devonian -or Mississippian carbonate rocks-Con. Previous work ...... 2 Unit 2 ...... -...... 30 Present work ...... :...... 3 Unit 3 ...... ·.-...... :...... 31 Mississippian carbonate rock...... :...... 31 ~~~:~e::::;~ s~W~~~~~~~~~~~~~~~~~~~~~~~~~-~~~~~~~~~~::::::::::::::::::: : Paleozoic carbonate· rocks, undivided...... 32 Precambrian rocks ...... •...... -...... 4 Mesozoic plutonic rocks ...... _33 Belt Supergroup ...... 4 Flowery ·Trail Granodiorite...... :...... 34 Prichard Formation ...... 4 Starvation Flat Quartz Monzonite...... 38 Ravalli Group ...... 6 Phillips Lake· Granodiorite (and associated dikes).... 40 Burke Formation ········"···················· ...... 6 Two-mica quartz monzonite...... 44 Revett Formation ~--·····---~---································· 7 Coarse-grained quartz monzonite...... 45 St. Regis Formation...... :. 8 Muscovite quartz monzonite...... 46 Wallace Formation ...... 9 Cenozoic plutonic rocks...... ~...... 47 Missoula Group ...... 11 Silver Point Quartz Monzonite...... 47 Striped Peak Formation ...... 11 Granodiorite ...... 50 Membera ...... 12 Fine-grained quartz monzonite ...... 50 Memberb ...... 12 Differentiation of the plutonic rocks...... ~...... 51 Member c ...... 13 Potassium-argon ages of the plutonic rocks, by Joan C. Memberd ...... 16 Engels...... 52 Deer Trail Group ...... -...... 16 Togo(?) Formation ...... :...... 17 Cenozoic hypabyssal, volcanic, and sedimentary rocks...... 58 Edna Dolomite ...... ,...... 17 Mafic dikes ...... 58 McHale Slate ...... 17 Andesite ...... 60 Stensgar Dolomite ...... 17 Basalt...... 60 Buffalo Hump(?) Formation ...... ,...... 18 Latah Formation ...... ~...... 61 Deer Trail Group, undivided...... 18 Conglomerate ...... 61 Relation between the Belt Supergroup and the Glacial, alluvial, and talus deposits, undifferentiated.... 61 Deer Trail Group...... 18 Structure ...... 62 Metamorphic rocks, undivided ...... 24 Structures in the Belt Supergroup block...... 62 Windermere Group ...... 24 Folds ...... 62 Huckleberry Formation ...... 24 Faults ...... 64 Monk Form_ation ...... 26 Structures in the Deer Trail Group biock...... 65 Paleozoic rocks ...... 27 Other differences between the two blocks...... 66 Addy Quartzite ...... -...... 27 .Structure separating the two blocks...... 67 Carbonate rocks ...... 29 Other faults ...... 69 Metaline Formation ...... 29 Mineral deposits ...... 69 Devonian or Mississippian .carbonate rock...... 30 References. cited ...... 71 Unit 1 ...... 30 Index ...... -...... 73

ILLUSTRATIONS

Page PLATE 1. . Geologic map of the north half of the Chewelah-Loon ·Lake-area, Stevens and Spokane Counties, Washington ...... In pocket 2. Geologic map .of. the .south .half.of.the.. Chewelah-.Loon.. Lake area,.Stevens-and Spokane Counties, Washington ...... , ...... In pocket

FIGURE 1. Index map showing location of Chewelah-Loon Lake area ...... 2 2. Map showing location of some geographic features referred to in the text...... :...... -...... 3 3. Generalized columnar section of the Prichard Formation ...... :...... 5 4 ..- Generalized, columnar.. section of .member. c of, -the· Striped Peak -Formation ·on Quartzite ·Mountain...... 14

III IV CONTENTS

_ Page FIGURE 5. Photograph showing argillite and breccia from member c of the Striped Peak Formation...... 14 6. Photograph showing soft-sediment breccia in the McHale Slate, probable correlative of member c of the Striped Peak Formation ...... 15 7. Columnar sections showing possible correlations between the Deer Trail Group and the Belt Supergroup...... 21 8. Generalized geologic map showing the distribution of the Belt Supergroup and Deer Trail Group in and around the Chewelah-Loon Lake area···············-··········································································································· 22 9. Photograph showing the bold cliffs (dip slopes) of Addy Quartzite which form the west side of Quartzite Mountain ...... 27 10. Photograph showing the contact between the Addy Quartzite and member d of the Striped Peak Formation...... 27 11. Ternary plots of modes of plutonic rocks ...... _...... 34 12-15. Photographs showing: 12. Fine-grained dike-fomi mass of graphically intergrown rock that intrudes the Starvation Flat Quartz Monzonite ...... 40 13. Stained slab of Phillips Lake Granodiorite._...... 41 14. Stained slab of Silver Point Quartz Monzonite and granodiorite ...... 48 15. Silver Point Quartz Monzonite...... 49 16. Map showing potassium-argon sample localities ...... ~---·····································-~---······························································ 54 17-21. Graphs showing: 17. Changes in apparent ages of hornblende, biotite, and muscovite in an older pluton intruded by a younger one ...... ·······················-··········································································································· 55 18. Apparent ages of hornblende and biotite from the Flowery Trail Granodiorite...... 55· 19. Apparent ages of muscovite and biotite from the Phillips Lake Granodiorite and associated dikes ...... 56 20. Relation of apparent ages to concentration of clean-clear hornblende in the fine-grained quartz monzonite ...... ;...... " 57 21. Plots of mafic dike modes...... 58 22. Map showing location of major fold axes in the Belt Supergroup block ·············'·························································· 63 23. Sketch map showing alternate interpretations of the relation of the Addy Quartzite to the northwest~trending faults between Eagle Mountain and Jumpoff Joe Mountain...... 64 24. Cross sections showing possible interpretations of thrust relations between the Belt Supergroup and Deer Trail Group ...... _...... 68

TABLES

Page TABLE 1. Chemical analyses and CIPW norms of basalt and one tuffaceous. rock from greenstone of the Huckleberry Formation ·································~·········································································································· 25 2. Petrographic data on plutonic rocks...... 35 3. Potassium-argon data on plutonic rocks...... 53 4. Mineral composition of selected mafic dikes..... _...... 59 GEOLOGY OF THE CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

By FRED K. MILLER and LORIN D. CLARK

ABSTRACT N. 30°-40° E. The consistency of the different trends of and The report area, two 15-minute. quadrangles, is in the south­ in, the two blocks and the apparent differences in facie~ and ern part of Stevens County and the northern part of Spokane thickness of probably equivalent rocks in each block suggest County, about 30 miles north of Spokane. Much of the area is that the blocks have been juxtaposed along a thrust fault. underlain by two great Precambrian sections-the Belt Super­ High-angle faults, which are found in both structural blocks, group and the Deer Trail Group. The Deer Trail Group strike about N. 50°-60° E. and have displaced rocks on the appears to be equivalent to the upper part of the Belt Super­ south sides downward relative to the north sides. These faults group, although differences in thickness and stratigraphy sug­ may be contemporaneous with a set of approximately north­ gest that the sites of deposition of the two sections were much south-striking faults which are inferred along the east side of farther apart than the sections are now. The Precambrian the Colville River valley. Huckleberry Formation and Monk Formation unconformably The major thrusting is interpreted as having taken place less overlie the Deer Trail Group, but not the Belt Supergroup. than 100 million years ago. The inferred north-south faults on the east side of the Colville River valley appear to cut the 50- ~oth Precan:tbri~.n .group~. ~~e. ove~lain by the .C~brian .~ddy Quartzite. Cambrian, Devonian, and Mississippian carbonate million-year-old Silver Point Quartz Monzonite but do not rocks are found.above.the Addy Qua.rtzite where.it overlies the offset the Miocene and Pliocene Columbia River Group. Belt Superg~oup. · .. · · · · · · · .. ,.. · · · · · ~!n~pluton~, representing three periods of plutonic activity, INTRODUCTION intrude the Precambrian and Paleozoic rocks. They range in composition from granodiorite to alkali-rich quartz monzonite. LOCATION. AND ACCESSmiLITY The old~~t J~.ttte ~Fl.owery Tr~l9ra.n9~~orite, an isolated pluton near the center of the area. It appears to have been intruded The report area, which coincides with the east half about 200 million years ago. Five others, mainly in the northern of the Chewelah 30-minute quadrangle, covers about part of the area, were intruded about 100 million years ago, and 400 square miles of Stevens and Spokane Counties in intrusion of the remaining three, the Silver Point Quartz Mon­ northeastern Washington (fig. 1). Although the 30- zonite and two small satellitic plutons in the southern part of the area, apparently climaxed plutonic activity about 50 million minute topographic map is now out of print, 7% -minute years ago. topographic coverage is available for the entire quad­ Andesite of Oligocene(?) age occurs in one small area south­ rangle. The north half of the report area comprises the west of Chewelah. Yakima-type(?) flows of the Columbia River Calispell Peak, Cliff Ridge, Chewelah, and Goddards Group are preserved at lower elevations in the southern part Peak 7%-minute quadrangles, and the south half the of the area. Small patches of conglomerate, possibly of Tertiary age, unconformably overlie the Huckleberry -Formation south­ Nelson. Peak, Valley, Springdale, and Deer Lake quad­ west of· Chewelah· and the Starvatioii"'Flat "QuartzMOnZoiiite rangles. In addition, the north half is topographically near Cliff Ridge. Quaternary giaci.8"1 ~~ci ·~ifu~f;J-deposi~~~~~r mapped at a scale of 1:62,500 and is named the Chewe­ large parts of the west half of the area at lower elevations. lah Mountain quadrangle. The Belt Supergroup and the Deer Trail Group appear to Figure 2 shows the location of the report area in be confined to different structural blocks that are separated by a major structural discontinuity. The chief structures of the relation to Spokane, the nearest major city, and in Belt Supergroup block, which underlies most of the eastern addition shows many of the geographic features fre­ part of the area, are a roughly north-south-striking anticline quently referred to in the text. South of Chewelah, the and syncline. Both folds are overturned to the west in the west edge of the area follows the west edge of the Col­ northern part of the area. Six large northwest-striking high­ ville Valley, and the east border roughly follows the angle faults appear to predate the folding and are probably Precambrian. Although the sense of movement on these faults divide separating the Colville and Pend Oreille River is not well established, five show apparent left-lateral slip, and valleys. one shows apparent right-lateral slip. The five could well be Most of the roads in the area are unsurfaced. Numer­ dip-slip faults that have displaced rocks on the north sides ous county, logging, and mining dirt roads provide easy downward relative to the south sides. access to all parts of the area except the northeast The north-south folds and northwest faults of the Belt Super­ group block are apparently truncated by the Deer Trail Group corner. The Burlington and N orthem Railroad links block, which underlies the western part of the area and strikes Chewelah, Valley, Springdale, and Loon Lake with 1 2 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

EXPLANATION ~ C9mpleted work f0:0:l ~ Work in progress

1. Togo Mountain quadrangle- R. C. Pearson 2. Orient area- Bowman (1950) 3. Northport area- R. G. Yates 4. Deep Creek area- Yates (1964) 5. Kettle Falls area (west of river) -C. D. Campbell 6. Kettle Falls area (east of river)- J. W. Mills 7. Colville area- W. A. G. Bennett 8. Twin Lakes quadrangle- G. E. Becraft 9. Inchelium quadrangle- A. B. Campbell 10. Bead Lake area- Schroeder (1952) 11. Wilmont Creek quadrangle- Becraft (1966) 12. Hunters quadrangle- Campbell and Raup (1964) 13. Turtle Lake quadrangle·- Becraft and Weis ( 1963) 14. Mt. Spokane quadrangle- A. E. Weissenborn 15. Greenacres quadrangle- Weis (1968) 16. Metaline area- Dings and Whitebread ( 1965) 17. Magnesite belt - Campbell and Loofbourow 118° (1962) ___ 18. Newport 30- minute quadrangle (includes remapping ~, ~48° area 10)- F. K. Miller

l WASHINGTON 1 -~ _____ ..... -J

0 10 20 MILES

FIGURE 1.-Location of the Chewelah-Loon Lake area and surrounding areas for which geologic maps at a scale of 1: 62,500 or larger are available or mapping is in progress. Date indicates published report available; see list of references in text.

Spokane to the south, and Nelson, British Columbia, clastic and carbonate rock beneath the Huckleberry to the north. Formation. Besides mapping in the magnesite belt, PREVIOUS WORK Bennett prepared a map of the south half of the Colville ·• 30-minute quadrangle, a generalized version which Weaver (1920), in a comprehensive report on the appeared in Mills and Yates' report on high-calcium mineral resources of Stevens County, published the limestone (1962, pl. 6). first geologic map of the area and attempted to estab- Campbell and Loofbourow ( 1962) prepared a lish a stratigraphic section. Jones (1928) mapped the detailed geologic map of the entire magnesite belt and Chewelah 30-minute quadrangle in considerably more assigned all rocks beneath the Lower Cambrian Addy detail than Weaver. He retained most of Weaver's units, Quartzite to the Precambrian. They also compared the differentiated several more, and recognized the con- rocks of the Deer Trail Group with those of the Belt glomerate at the base of the Huckleberry Formation. Supergroup (p. F20). Studies since Jones' have concentrated mainly on the Campbell and Raup (1964) mapped the Hunters northeastern Washington magnesite belt, immediately quadrangle, which covers most of the southwestern part west of the report area (fig. 2). The magnesite belt is of the magnesite belt. Their map, although at a slightly underlain by many of the Precambrian and lower Pale- smaller scale than Campbell and Loofbourow's, is more ozic units found in the report area. detailed. Bennett (1941) prepared the first good geologic map Schroeder (1952) mapped an area about the size of of the magnesite belt. He improved on earlier struc- a 15-minute quadrangle around Bead Lake, east of the tural and stratigraphic interpretations, and he assigned report area (fig. 2). He correlated parts of a thick sec­ a Precambrian age to the thick section of fine-grained tion of quartzites, argillaceous sandstones, and argil- INTRODUCTION 3

BEAO LAKE : AREA I

CLARK FORK QUADRANGLE

CLAYTON UADRANGLE

WEST END OF COEUR

0 10 20 MILES D'ALENE DISTRICT

FIGURE 2.-Location of some geographic features referred to in the text. lites there with the Prichard Formation and the Ravalli with the help of J. C. Moore. Mapping of the southern Group of the Belt Supergroup. This section of Belt 15-minute quadrangle was begun by Miller in 1966, rocks is the nearest one to the report area that has been continued with the help of R. C. Reynolds in 1967, and relatively well studied. Pre-Tertiary sedimentary rocks completed in 1968. Two preliminary geologic maps have known to exist between the report area and the Bead been published by the Washington Division of Mines Lake area probably consist entirely or in part of Belt and Geology (Clark and Miller, 1968; Miller, 1969). rocks also (A. B. Griggs, oral commun., 1967). ACKNOWLEDGMENTS PRESENT WORK The authors would like to thank Mr. PhillipP. Skok Work in the report area began in June 1962 as for his cooperation and help during the study of the a cooperative project of the Division of Mines and mines in the Eagle Mountain area. Mr. ·Skok, repre­ Geology of the Washington Department of Natural senting several of the mine owners, very generously Resources and the U.S. Geological Survey. The authors allowed many of the company mine maps to be dupli­ mapped most of the Chewelah Mountain quadrangle cated in the Chewelah Mountain quadrangle prelimi­ in 1963 and 1964. Miller finished the mapping in 1965 nary report. 4 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON We are particularly indebted to Allan Griggs, of the PRECAMBRIAN ROCKS U.S. Geological Survey, who introduced us to the Belt Supergroup in the Coeur d'Alene district and then BELT SUPERGROUP helped with many of the stratigraphic problems we The Belt Supergroup is a well-known section of Pre­ encountered in the Belt Supergroup in the Chewelah­ cambrian sedimentary rocks extensively exposed in Loon Lake area. We are grateful for the professional western Montana, northern Idaho, and eastern Wash­ courtesies extended by Marshall Huntting, W. A. Ben­ ington. In Canada it is known as the Purcell Series. nett, and .Jerry Thorson of the Washington Division It is generally divisable into a carbonate-rich eastern of Mines and Geology, who helped in all aspects of the facies and a more clastic western facies. Many forma­ project from the fieldwork to the final editing. Bennett's tions, some of which are correlative, have been named intimate knowledge of the region saved both authors' and delineated by various workers. The large number time and effort on more than one occasion and several of Belt units, confusing to many who have not studied times directed us to pertinent features we might other­ these rocks, is an indication of the complexity of the wise not have found. Finally, we would like to express internal stratigraphy of the group. Detailed strati­ our appreciation to Robert G. Yates and the late Arthur graphic descriptions and measured sections of the Belt B. Campbell, of the U.S. Geological Survey, who helped rocks are difficult or impossible to obtain in the report us to become acquainted with the regional geology of area because of poor exposures and extensive· forest northeastern Washington. cover. The report area contains the westernmost exposures REGIONAL GEOLOGIC SE'ITING of the Belt Supergroup. If the Deer Trail Group is cor­ relative with part of the i3'eit; iloW"evei, tile~i3-eit terrane In an excellent summary of the regional geology of extends abouf15 mlies fiuther west. - . . northeastern Washington, northern Idaho, and north­ western Montana, Yates, Becraft, Campbell, and Pear­ PRICHARD FORMATION son (1966, p. 47) recognized an eastern and a western geologic province, divided by the Columbia River val­ DISTRIBUTION AND TOPOGRAPHIC EXPRESSION ley. They defined the eastern province as characterized The Prichard Formation is a thick sequence of inter­ by Precambrian Belt and Windermere rocks and mio­ bedded argillite, siltite, and fine-grained quartzite. It geosynclinallower and middle Paleozoic rocks, and the underlies 38 square miles in the report area. An addi­ western province as composed of eugeosynclinal upper tional 12 square miles is covered by surficial deposits Paleozoic and Mesozoic rocks. The report area lies or occupied by younger plutons. About 13,000 feet of about 15-20 miles east of the boundary between the the formation is exposed in the core of a large, highly two provinces. faulted anticline, which occupies the entire east-central Both provinces are underlain by numerous bodies of part of the report area. Only the west half of the anti­ Mesozoic plutonic rock, which Yates, Becraft, Camp­ cline is found here, but reconnaissance of the rocks to bell, and Pearson ( 1966, p. 56) divided into two major the east suggests that Belt strata continue in that batholiths. As they have defined the boundaries of these direction and that much of the other half may have batholiths, the report area includes parts of both batho­ been replaced by plutons. The southeast limb of the liths. Most of the extensive basalts of the Columbia fold may be exposed along the west shore of Horseshoe -~. River Group: lie south of the report area, although a few Lake about 7 miles east of Deer Lake. There, a thick prongs extend into the southern part. section of dark laminated argillite, strongly resembling Broad open folds and high-angle normal faults which the upper part of the Prichard Formation, strikes north­ trend north-northwest are found in the Montana part east and is truncated by plutonic rocks just north of of the eastern province. The complexity of the structure the lake. increases towards the west, and in Idaho west-north­ The Prichard Formation was originally named the west-trending strike-slip faults and locally overturned Prichard Slate by Ransome (1905, p. 280-281) for the folds appear (Yates and others, 1966, p. 52). exposures along Prichard Creek in the Coeur d'Alene In northeastern Washington, the folding and faulting district, Idaho, where the unit, consisting chiefly of are~ noHceabiy- more- -complex -and irregular. In the argillite and well-indurated sandstone, was estimated report area, strike~slip' faults~ thrust faults; and'locally, to be more than 8,000 feet thick. The base has not been large-overturnedfofdsare' cut -by norllieas't- and north­ found in the report area or anywhere else. · stnldng hlgh~angle· dip-slip-· faults: . The- major move­ The formation forms much of the higher mountain­ me.iitS-iirohabhi ·occurred iii the .. Precainhria.n,' middle ous country along the east border of the area. Although and late Mesozo~c, and ea_riy Tertiary. - - . . -- . . - they do not crop out as well a~ the argillite, the quartz- PRECAMBRIAN ROCKS 5 itic parts of the formation are probably responsible for Siltite of the rugged topography formed by the unit. The road Burke Formation along the high ridge crest between Chewelah Mountain and Nelson Peak traverses quartzite for much of .its length. Joint-bounded blocks of all sizes litter the areas underlain primarily by quartzite, but almost all have been heaved, rotated, or otherwise moved by surface ru or near-surface processes. In-place outcrops are abund­ .c E ant in areas underlain by predominantly argillitic parts Cll of the formation, but topographic expression is more E... Clla. EXPLANATION subdued. a. :::> STRATIGRAPHY AND THICKNESS

The Prichard Fonilation can be divided into at least Argillite three members in the report area. Although they have not been distinguished on the geological map because of lack 9f time, for descriptive purposes they are inform­ ally referred to as the upper, middle, and lower mem­ Siltite bers in the following paragraphs. The upper member ... I. ·. · ...... ·.·.·.·.·.· J Cll .c ;· ~ ... :::· :: ~ ;·: .·.. ·:: :· :,. ·:.. .:· :. ;': ·:.:; (fig. 3) is predominantly dark-gray to black laminated E : : : Cll argillite, with a lesser amount of siltite, and contains E Quartzite Cll only an occasional bed of quartzite. The middle member 15 consists of interbedded siltite, quartzite, and minor ~ ~ argillite. The lower member probably consists of about 6,000 feet of interbedded siltite and argillite, although information is available only from sparse outcrops and FEET 4000 float. The upper member is locally divisible into three sub­ units, in descending order: 600 feet of argillite, 800 feet 3000 of siltite, and 3,000 feet of argillite. All the argillite in the upper member is well bedded to laminated and light to dark gray; locally it contains thin bleached zones, ,_ 2000 Cll This part of sec­ whose shape and orientation seem to be controlled by .c tion poorly E bedding. Very little quartzite or siltite is interbedded Cll known and within any of the argillite of the upper member. The E poorly exposed, but float con­ 1000 siltite subunit is fairly distinctive owing to its color, sists of same re­ gray to dull yellowish gray, and its thick indistinct petitive lithol­ bedding, vivid contrasts with the relatively dark lami­ ogies as found nated argillite zones above and below. The subunit also above 0 includes a number of quartzite and argillite beds. The· lower argillite subunit is generally darker than the upper one. It is the thickest relatively pure argillite zone in the report area. Base not The transition into the quartzitic middle member of exposed . the Prichard Formation begins about 4,400 feet from FIGURE 3.-Generalized columnar section of the the top of the formation, where quartzite and siltite Prichard Formation in the Chewelah-Loon Lake area. Lower, middle, and upper members beds start becoming thicker and more numerous (fig. are informal subdivisions and are not shown 3). The base is also a transition zone. The average on the geological maps. thickness of the member is about 3,200 feet thick. Although it probably consists of about 50 percent lighter colored than the argillite of the upper member. quartzite, numerous zones within the member are simi­ The quartzite is light gray to white, fine gt·ained, and lar to the transition zone at the top. Relatively pure vitreous. ·All gradations from quartzite to white siltite argillite zones as much as several hundred feet thick are found. Beds of both quartzite and siltite average also are found in this member, but most are distinctly between 1 and 5 feet in thickness and weather out in 6 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON large joint-bounded blocks. In general the more siltitic Mountain. This sill may be discordant, as it appears to to argillitic the rock, the thinner the beds. be higher in the section in the northern part of the area. The lower member only occurs east of the divide that It is difficult to be certain, however, because of the connects Chewelah Mountain and Nelson Peak. Scat­ strong overturning in this area and because of the possi­ tered outcrops, a few exposures in logging roadcuts, and bility of additional undetected structures. sparse float suggest that it is composed of the same recurring litholoii.es as the other members but that RAVALLI GROUP siltite and argillite are predominant. BURKE FORMATION The top of the Prichard Formation in the report area is defined as the top of the highest 600-foot-thick argil­ DISTRIBUTION AND THICKNESS lite zone beneath the quartzite, siltite, and argillite The Burke Formation consists primarily of light-gray transition beds of the Burke Formation. This is con­ siltite with lesser amounts of argillite and quartzite. It trary to the placement of the Prichard-Burke contact crops out in patches along a belt extending from east in most other Belt sections and should be taken into of Deer Lake northward to the South Fork of Chewelah consideration when the thicknesses qf_ ~he Prichard and Creek. The belt is offset by many northwest-trending Burke Formations are compared with those in other faults. sections, especially the relatively near well-studied The Burke Formation was named and fir~t described sections around Pend Oreille Lake and in the Coeur by Rans~me (1905, p. 280-281) and Ransome and d'Alene district. The upper part of the Prichard Forma­ Calkins (1908, p. 32) for the exposures around the tion in the report area appears to pe more akin to that town of Burke in the Coeur d'Alene district. They at Pend Oreille than that around Coeur. d'Alene. reported a section about 2,000 feet thick "composed Most workers agree that the bulk of the Belt Super­ of rocks that show all gradations from nearly pure group was deposited in shallow water (Ransome and quartzite to siliceous shale." Hobbs, Griggs, Wallace, Calkins, 1908, p. 17; Yates and others, 1966, p. 4 7; and Campbell (1965, p. 34) reported a similar thickness Hobbs and othe~, 1965, p. 33). Hobbs, Griggs, Wallace, but also found that it varied from place to place in the and Campbell ( 1965, p. 33) interpreted the relatively district, generally thickening to the west. clean, well-sorted quartzites at various levels in the sec­ The Burke Formation averages about 3, 700 feet in tion to be suggestive of a nearshore environment. Others thickness in the report area. Miller (1969, p. 1) errone­ have pointed out that· most of the sedimentary struc­ ously reported an average thickness of 4,500 feet, which tures in the Prichard Formation are confined to the was calculated for the thickest exposed section of the uppermost part (Campbell, 1960, p. 552; Campbell and formation, located along the margin of the major valley Good, 1963, p. A8; Hobbs and others, 1965, p. 34). northeast of Deer Lake. Other thicknesses have been In the report area, both ripple marks and mud cracks, calculated; the minimum, 3,100 feet, was calculated although not abundant, have been observed in the for a section north of Goddards Peak. Although some upper part of the highest member. Cross laminations thickness variation may actually exist, most of it is due and graded bedding on a fine scale are found through-.· to differences in where the contacts are placed, especi­ out the formation but are quite ·subtle. The pyrite ally the upper one. and pyrrhotite and accompanying stained surfaces described by almost all investigators are also found in STRA TIGrt.APHY the report area. The Burke consists mostly of light- to medium-gray Several sills- of greenish-black amphibolite are found siltite. The basal zone, 150 feet of alternating argillite in the middle and lower members of the formation. and siltite, forms a transition between the argillite of They have been thoroughly recrystallized, probably the underlying Prichard and the siltite of the Burke. from diabase, and the primary texture has been com­ About 500 feet above the contact, the siltite grades into pletely destroyed. Although they appear to have caused a quartzite zone approximately 300 feet thick. This only slight recrystallization of the host rocks, they seem quartzite zone, in tum, grades upward into a predom­ to have "set up" the rocks for metamorphism by later inantly siltite section. Zones of quartzite or argillite as intrusive bodies. In the vicinity of younger plutonic much as a few tens of feet thick are interlayered with rocks, the siltite and argillite adjacent-to the sills show the siltite, mostly in the lower half of the formation. pronounced recrystallization two to three times as far The upper 400 feet is the only other part of the unit from the plutonic contact as other rocks of comparable that contains a significant proportion of quartzite. composition not located near the sills. Most of the sills About 3,000 feet above the base is a distinctive 100- appear to be strikingly concordant, with the possible 300-foot-thick zone of maroon to lavender argillite, exception of the large body northeast of Chewelah siltite, and quartzite, an almost perfect duplicate of PRECAMBRIAN ROCKS 7 typical St. Regis lithology. The argillitic parts of the and terminates against a crosscutting intrusive body. zone contain mud cracks, mudchip breccias, and ripple The Revett Formation was initially named the Rev­ marks. The quartzite is very fine grained and in places ett Quartzite by Ransome (1905, p. 280-281) and coarsely laminated. Laminations average about an Ransom and Calkins ( 1908, p. 35) in the Coeur d'Alene eighth of an inch thick and exhibit fine planar cross­ district, Idaho. Harrison and Campbell (1963, p.1418), bedding internally. in a discussion of correlation problems within the Belt Much of the Burke Formation, particularly the Supergroup, noted that the Revett is no more homo­ siltite, is medium to lig~t gray with a subtle, barely geneous than any other Belt formation in northern noticeable purple cast. Some of the argillite beds in the Idaho or western Montana and suggested that the uppermost and lowermost parts of the Burke Forma­ name be changed to Revett Formation. Their observa­ tion have a gray-green cast, as does some of the siltite. tion regarding the inhomogeneity of the quartzite is This coloration is restricted to specific parts of the quite apparent in the report area, and the new name Burke here but appears to be typical of much of the they suggest will be used in this report. formation at Coeur d'Alene (Hobbs and others, 1965, Poor exposures prevent detailed measurement of any p. 35) and Pend Oreille (Harrison and Jobin, 1963, p. Revett sections in the report area, although from out­ K10). The light-weathering rind on Burke siltite crop width the thickness of the formation appears to described at both of the just-mentioned localities is vary little from an average of about 3,100 feet. As present in the report area but is not nearly as well Hobbs, Griggs, Wallace, and Campbell (1965, p. 37) developed nor as extensive. pointed out, however, at least part of the discrepancies Beds range in thickness from less than 1 inch to more in thickness of the Revett Formation from place to than 10 feet but commonly are about 6-12 inches thick. place may be due to differences in the way that its con­ In general, the finer grained rocks form the thin beds, tacts have been placed by different workers. and the relatively coarser grained rocks the thick beds. Argillite, particularly in the uppermost and lowermost STRATIGRAPHY AND LITHOLOGY parts· o£ the formation, appears to constitute a larger Most of the Revett Formation in the report area is proportion of the section than it actually does. This composed of fine-grained conspicuously clean looking is because thin argillite beds and partings form. skins vitreous quartzite. Although, as Harrison and Campbell on large exposures of bedding surfaces. pointed out, the Revett Formation is probably as heter­ Oscillation and current ripple marks are locally ogeneous as any other formation in the Belt Super­ abundant in the upper part of the formation. Some group, the clean looking quartzite beds that character­ quartzite strata at various horizons are crossbedded, ize the Revett contrast rather strikingly with the pre­ and much of the siltite is finely cross laminated. In dominantly impure clastics of the other formations. some layers the fine but somewhat indistinct lamina­ In the report area, the depositional boundaries be­ tions in the siltite are broken and disrupted, presum­ tween all formations are transition zones, rather than ably by slumping or some other form of contemporane­ sharp contacts. The transition zone between the Burke ous deformation. Formation and the overlying Revett Formation is the The mineralogy of the Burke Formation, like that of widest in the Belt Supergroup in the report area. The the other Belt units, is relatively simple. Quartz, seri­ same criteria used by Hobbs and others (1965, p. 35) cite, plagioclase, potassium feldspar are the most com­ to draw the contact in the Coeur d'Alene cij.strict are mon minerals, generally in that order of abundance. used in this area; the base of the Revett Formation is Zircon, the most common heavy mineral, is found the horizon above which beds of vitreous quartzite pre­ throughout the formation. Small magnetite crystals dominate over siltite beds.· In the south half of the are also common. area, the transition zone has been reported to be about 200 feet thick (Miller, 1969, p. 1). In the northern half, REVE'IT FORMATION where the contact zone is better exposed, as on Jay DISTRIBUTION AND THICKNESS Gould Ridge east of Chewelah, this zone is closer to 400 feet thick. The Revett Formation is predominantly white to The lower 200-30.0 feet of the Revett Formation light-gray fine-grained quartzite. Within the report contains a fair proportion of siltite beds. These beds area, the distribution of the Revett is similar to that decrease upward in number and, less consistently, in of the Burke Formation. It parallels the latter and is thickness. Above the siltitic base is-about 1,000 feet of disrupted about equally by the same cross-faults. East relatively pure vitreous quartzite. Even in this interval, of Deer Lake, the formation continues to the southeast, however, there are both single and multiple beds of but a few miles outside the area it bends to the east siltite. 8 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON Near the middle of the formation is about 500 feet in argillaceous beds on the mountain north of Deer of interbedded siltite, siltitic quartzite, and quartzite. Lake, but these appear to be local. The rocks of this zone are almost identical with the · bulk of the Burke Formation and, lacking continuity ST. REGIS FORMATION with the more distinctive rocks above and below, could DISTRIBUTION AND THICKNESS easily be erroneously assigned to the Burke Formation. Just below this zone is 50-100 feet of dark, banded The St. Regis Formation consists of maroon and quartzite, some of which shows signs of crossbedding. green interbedded argillite and siltite. Numerous This quartzite is fine grained but coarser than most quartzite beds are interlayered with the finer grained in the formation. rocks near the base. Good outcrops of the St. Regis The middle siltitic part of the Revett Formation is Formation occur in only a few places in the large area overlain by a thick quartzite zone that contains rela­ underlain by the formation. Its distribution is roughly the same as that of the Revett and Burke Formations tively few siltite beds. About 200 feet from the top of ' the formation siltite beds are again noticeable and except in the area around Wilson Mountain and the become progressively more abundant toward the con­ area south of the Flowery Trail Granodiorite. Around tact with the overlying St. Regis Formation. In this Wilson Mountain the formation is folded into a strongly upper 200-foot interval, the quartzite and siltite have overturned syncline. The dips of both normal and over­ a slight lavender tint, making the gradation into the turned limbs are fairly low. The St. Regis Formation overlying St. Regis not only one of grain size but of is relatively thin for a Belt unit, but its outct·op width color also. is about 2 miles in that area because of the low dip of Beds range in thickness from less than 1 inch to more the beds and the relatively subdued topography. (See than 20 feet; average thickness is about 1~2 feet. Exten­ cross section A-A', pl. 1.) From the south margin of sive talus composed of quartzite blocks commonly the Flowery Trail Granodiorite to Grouse Creek, most accumulates at the foot of slopes underlain by the of the St. Regis Formation is covered by glacial debris. Revett Formation. In the report area, the average thickness of the St. Most of the quartzite is light gray to white. Siltite Regis Formation is about 1~600 feet. No sections were beds commonly are the same color, but many are darker measured, and at only a few localities are both contacts gray, similar to the color of those in the Burke Forma­ sufficiently well located so that a reasonably accurate tion. Thin bands of opaque minerals impart a faint thickness can be calculated. ·banding to the quartzite but are not abundant. Dissem­ Ransome ( 1905, p. 280-282) named and first inated dark minerals are common in most of the quartz- described the St. Regis Formation in the Coeur d'Alene ite but do not materially affect the color of the rock. district. Because of the excellent cliff exposures there, In places, quartzite beds contain pale-yellow roughly Calkins (in Ransome and Calkins, 1908, p. 3.7) was spherical pea-sized areas in which part of the cementing able to accurately measure the thickness of the forma­ agent of the rock has been removed. Similar features, tion. In the east-central part of the district he measured mentioned by Hobbs, Griggs, Wallace, and Campbell 996 feet ·of section but mentioned that the formation ( 1965, p. 38), appear to be more characteristic of the appears to be thicker to the east and to the south. formation in the Coeur d'Alene district than in the Hobbs, Griggs, Wallace., and Campbell ( 1965, p. 39) report area. The weathered quartzite surfaces have ··a measured 1,400 feet for the same section but included pock-marked appearance where these small vuglike a green quartzose argillite zone in the St. Regis Forma­ features have been leached out by surface waters. tion which Calkins put in the overlying Wallace For­ The siltite of the Revett Formation has virtually the mation. In the report area, the green beds · are also same mineralogy as the Burke Formation. The quartz­ included in the St. Regis Formation. ite is relatively pure but contains an unusually large According to Harrison and Jobin (1963, p. K12), the amount of heavy minerals, as much as a half of a per­ St. Regis Formation is from 600 to 1,100 feet thick in cent zircon and magnetite. Sericite and plagioclase are the Clark Fork quadrangle, 50 miles east of the report the most common impurities, in that order of abund­ area. However, they assigned the distinctive green argil­ ance. Grains of potassium feldspar are common but not lite beds, which are about 200 feet thick there, to the abundant. lower Wallace Formation; thus the range in thickness Crossbedding and ripple marks are the only common in the Clark Fork quadrangle can be increased to 800- sedimentary structures in the Revett Formation within 1,300 feet. the report area. Neither structure appears to be as well STRATIGRAPHY AND LITHOLOGY developed or as abundant as in the Coeur d'Alene dis­ The St. Regis Formation is one of the more distinc­ trict. A few beds containing mud-chip brecci~ are found tive units in the Belt Supergroup because of its striking PRECAMBRIAN ROCKS 9 color. It is typically red-purple or lavender, except for In the open, south-plunging syncline on Blue Grouse several hundred feet of siliceous carbonate-bearing Mountain, the maroon color of the St. Regis is com­ argillite at ~he top of the section, which is a distinctive pletely bleached out in places. Along strike, the rocks yellow green. grade from maroon, through mottled white and maroon, The lower 50-200 feet of the St. Regis Formation is to white or pale green. The white and pale-green rocks transitional into the dominantly quartzitic Revett For­ are poorly indurated and altered looking. All lithologies mation below. The lower contact has been placed where are affected. The bleaching appears to be confined to a the red-purple argillaceous and siltitic rocks predomi­ restricted area of hydrothermal alteration, which affects nate over the relatively clean white and lavender rocks underlying about half a square mile. The same quartzite of the upper part of the Revett Formation. type of bleaching is present around some of the large Defining the contact in this manner makes its place­ quartz veins on Blue Grouse Mountain, Deer Lake ment relatively simple in the field because most of the Mountain, and Bald Mountain. quartzite beds in the lower part of the St. Regis Forma­ tion are much more highly colored. White quartzite WALLACE FORMATION and red-purple argillite and siltite beds are actually DISTRIBUTION AND THICKNESS interbedded in only about 50-100 feet of the transition zone. The Wallace Formation is a thick unit made up of The lower three-fourths of the formation consists of argillite, quartzite, siltite, and carbonate rock. Much of interbedded argillite, siltite, and quartzite. In general, the quartzite and siltite are carbonate bearing. The the finer grained.rocks are more highly pigmented. The distribution of the Wallace in the report area is consid­ quartzites are pale lavender, and the argillites deep red­ erably more irregular than that of most other Belt dish purple; the siltite ranges from one color to the formations. It forms a roughly linear but highly faulted other. In this part of the formation, the rocks grade belt around the margin of the anticline in the east half from predominantly quartzite and siltite near the base, of the area, but also underlies part of Deer Lake Moun­ to predominantly argillite and siltite in the upper part. tain and Loon Lake Mountain in the southern part. In The unit, as a whole, appears to be more quartzitic in the north half of the area, the formation forms part the report area than in the Pend Oreille or Coeur of an up-faulted block on the west flank of Eagle d'Alene areas. Mountain; the block extends northward to McDonald The upper quarter of the formation is composed of Mountain. alternating zones of red-purple clastic rocks and yellow­ The formation is divided into an upper and a lower green carbonate-bearing siliceous argillite, which are part in the report area. Hobbs, Griggs, Wallace, and 10-50 feet thick. The red-purple zones are siltite and Campbell (1965, p. 41) divided it in the same manner argillite and are restricted to the lower two-thirds of in the Coeur d'Alene district. Because the lower part this part of the formation. The yellow-green argillite of the formation forms fewer outcrops than any other in tum grades upward into the Wallace Formation, Belt unit in the report area, it is difficult to obtain an mainly through an increase in carbonate-bearing accurate estimate of its thickness. The average calcu­ quartzite and black laminated argillite beds. lated thickness of the lower part of the formation at the Individual beds range in thickness from 1 inch to homoclinal section of the west flank of Bald Mountain about 4 feet in the lower part of the formation. The is about 3,500 feet. This is probably as accurate a figure thicker quartzite beds are faintly banded by darker as can be obtained in this area. purple layers. The maroon argillite and siltite in the A complete section of the upper part of the Wallace upper part of the formation are very thinly bedded and Formation is not exposed in the report area, as a fault in places laminated. Almost all siltite and quartzite marks the upper contact everywhere except on Deer beds are separated by at least a film of red-purple argil­ Lake Mountain, and the base of .the upper part is not lite. Bedding characteristics of the middle part of the exposed there. The thickest preserved sections are on formation are not well known owing to poor exposure. McDonald Mountain, Eagle Mountain, and just east of Mud cracks and mud-chip breccias are abundant Quartzite Mountain. The section on McDonald Moun­ throughout the St. Regis Formation, especially near tain is not as thick as the map seems to show, because the top. 'Even where the unit is mapped largely on the about one-third to one-half of the outcrop width is basis of float, many chips and pieces of float have mud made up of sills and dikes of leucocratic quartz mon­ cracks on them. Ripple marks and cross-laminations zonite. A calculated thickness of 2,500-3,000 feet for are also common but not nearly as abundant as in the the section east of Quartzite Mountain probably repre­ Coeur d'Alene district, perhaps because exposures in sents a maximum for this part of the Wallace Forma­ one area are so much better than in the other. tion. That section also is fault bounded, although it 10 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

appears to be relatively undisturbed internally. On The unit is characterized by alternating beds of light­ Eagle Mountain, the upper part of the Wallace Forma­ colored quartzite and siltite and dark argillite. Carbon­ tion is highly sheared and may contain unrecognized ate minerals are confined mostly to the quartzite and faults. siltite. The thickness of these relatively coarse grained The formation was first named in the Coeur d'Alene beds is highly variable not only from bed to bed but district by Ransome (1905, p. 280, 282). Calkins (in within ·a single bed as well. The most striking lateral Ransome and Calkins, 1908, p. 40) estimated the thick­ changes in bed thickness appear to occur in carbonate­ ness there to be at least 4,000 feet, but he did not divide bearing quartzite, which, in the extreme, thins from 5 the formation. Hobbs, Griggs, Wallace, and Campbell feet to 'a few inches within a distance of less than 10 ( 1965, p. 42), who studied that area in greater detail, feet. Most of the quartzite beds are thinner, however, found no well-exposed uninterrupted sections of the and repeatedly pinch and swell. Hobbs, Griggs, Wal­ formation. From measurements of several partial sec­ lace, and Campbell ( 1965, p. 44) suggested that this tions they concluded that the total thickness is between boudinagelike appearance may be attributed to com­ 4,500 and 6,500 feet. paction adjustment and later deformation. The thicker STRATIGRAPHY AND LITHOLOGY quartzite and siltite beds show almost no internal stratification. Rarely do colored impurities define bed­ LOWER PART ding within these layers, although the upper part of a The lower part of the Wallace Formation is composed coarser grained bed commonly grades into an overlying of argillite, siltite, quartzite, highly impure carbonate argillite layer not only by a decrease in grain size but rock, carbonate-bearing quartzite, and carbonate­ more obviously by darkening in color. bearing siltite. It is varied in lithology, but is generally The argillite layers, which contain most of the abun­ easy to recognize not only because of the presence of dant sedimentary structures. in the lower parts of the carbonate-bearing rocks but because of bedding char­ Wallace Formation, are dark gray to black and com­ acteristics which are unique in the Belt Supergroup. monly finely laminated. ·Exposed bedding-plane sur­ The lower contact of the Wallace Formation is faces generally have a blue-black phyllitic sheen. Unlike assigned on the basis of criteria that Hobbs, Griggs, the very even continuous laminations in other Belt Wallace, and Campbell ( 1965, p. 43) used because formations, the laminations in this part of the section this part of the section is similar to that in the Coeur are commonly discontinuous, disrupted by other sedi­ d'Alene district. Hobbs drew the contact where the mentary structures, or disrupted by differential com­ alternating beds of quartzite and argillite of the Wal­ paction. Small discordant bodies of siltite or quartzite, lace Formation became more numerous than the lami­ which look like miniature clastic dikes or filled mud nated green argillitic rock of the St. Regis Formation. cracks, cut the laminated argillite, commonly at right In the report area, this transition zone is about 100 feet angles. These bodies are· generally less than 1 inch long thick and is well exposed below the upper Cottonwood and have been crenulated, probably by compaction. Road east' of Parker Mountain and in the streambed They are found throt,Ighout the Wallace Formation and of Grouse Creek north of Bald Mountain. in parts of the Striped Peak Formation, but are most Many of the transition beds are carbonate bearing, abundant in the lower part of the former. Desiccation · as are some of the beds for several hundred feet below mud cracks and, to a lesser degree, ripple marks are and several thousand feet above the transition. The common throughout the unit, and in places mud cracks lower part of the formation is one of the main carbon­ are superimposed on ripple marks. Crossbedding was ate-bearing units in the western Belt Supergroup, but not seen, although fine cross-laminations were found in in the report area almost none of this rock resembles some of the darker fine-grained siltite beds. normal limestone or dolomite because the carbonate minerals usually make up less than 50 percent (average UPPER PART is about 20 percent). Partial sections of the upper part of the Wallace As well as can be determined from the small isolated Formation crop out best on Deer Lake Mountain, the outcrops, the lower part of the formation appears to south flank of Parker Mountain, and the hill east of grade upward, with numerous local reversals, from pre­ Quartzite Mountain. At these localities the rocks are dominantly carbonate-rich quartzite and siltite to silt­ predominantly composed of dark laminated argillite ite or argillite and lesser amounts of quartzite. The but also contain zones of relatively pure looking limy rocks in the upper part of the lower Wallace, particu­ dolomite and beds of carbonate-bearing quartzite iden­ larly near the transition into the predominantly argil­ tical with those in the lower part of the formation. In litic upper Wallace, contain less carbonate minerals this respect the lithology of the upper part in this area than the lower part of the formation. is intermediate between that at Pend Oreille Lake and PRECAMBRIAN ROCKS 11 in the Coeur d'Alene district. Harrison and Jobin quartzite beds on the west flank grade upward into (1963, p. K13) divided the Wallace Formation into five typical black laminated argillite of the upper part of members in the Clark Fork and Packsaddle Mountain the formation. However, 200-300 feet above that the quadrangles near Pend Oreille Lake. There, the transi­ lowermost member of the Striped Peak Format~on rests tion from the lower calcareous member to the argillite on the black argillite. If that contact is depositional, as member, found about 2,500 feet above the base of the ·it appears to be, then the quartzitic rock exposed in formation, appears to represent roughly the same inter­ roadcuts along the south shore of the lake must be val as the transition from the lower part to the upper an anomalous zone in the upper part of the Wallace part of the Wallace Formation in the report area and Formation. in the Coeur d'Alene district. However, Harrison and Mud cracks, ripple marks, graded bedding, and, to Jobin ( 1963, p. K15) described additional beds resem­ a lesser degree, cross-laminations are found in this part bling those of the lower part in the member above the of the section, although none are extensively developed. argillite member, and a calcareous member above that, The crenulated clastic dikelets or crack fillings so which is composed principally of dolomitic limestone. abundant in the lower part of the formation are com­ Although Hobbs, Griggs, Wallace, and Campbell ( 1965, mon but much less numerous. Slump structures and p. 44) mentioned a few hundred feet of carbonate-rich preconsolidation deformation, which appear in some of beds in the upper part of the Wallace Formation, the the similar looking argillites in the Striped Peak For­ section they described at Coeur d'Alene is made up pre­ mation, are not found in the upper Wallace Formation. dominantly of dark laminated argillite. On Eagle Mountain, 3 miles northeast of Chewelah, MISSOULA GROUP the upper part of the Wallace Formation contains num­ STRIPED PEAK FORMATION erous beds, as much as 10 feet thick, of light-tan limy dolomite. In the two fault-bounded blocks on the north­ DISTRIBUTION AND COMPARISON WITH east flank of the mountain, the unit is made up of about OTHER NEARBY SECTIONS equal parts of black laminated argillite, laminated car­ The Striped Peak Formation in the report area is bonate-bearing argillite, and light-tan dolomite beds. confined for the most part to a narrow band on the Here the various lithologies are repeated in groups from east side of the Addy Quartzite, which trends roughly a few feet to a few tens of feet in thickness. The aggre­ north to south from Eagle Mountain to J umpoff Joe gate preserved thickness of this part of the unit is prob­ Mountain. Other outcrops are found on Deer Lake ably 800-1,000 feet. Although it is fault bounded, this Mountain and possibly beneath the Addy Quartzite on carbonate-rich zone probably occurs at least 3,000 feet the south flank of Blue Grouse Mountain. above the transition from the lower part of the forma­ The formation was originally named and described tion because the zone is not found in the intervening in the Coeur d'Alene district by Ransome ( 1905, p. 280, interval at the relatively well exposed section east of 282). Calkins (in Ransome and Calkins, 1908, p. 44) Quartzite Mountain only 4 miles to the south. estimated the thickness of the preserved section to be East of Quartzite Mountain the unit is exposed bet­ about 1,000 feet. Shenon and McConnel (1939, p. 5) ter than at any other place in the area and is fairly measured 1,500 feet of section about 1 mile southeast similar to the upper part in the Coeur d'Alene district. of Striped Peak, the type locality. It is predominantly dark-gray to black laminated argil­ Although not particularly thick, one of the best and lite. There are a few quartzitic zones as much as 50 most complete sections of the Striped Peak Formation feet thick which are made up of individual quartzite is well exposed in a large part of the Clark Fork quad­ or siltite beds as much as 1 foot thick. Lensoidal seams rangle, Idaho. Harrison and Jobin (1963, p. K16) of brown siltite averaging about 0.1 inch in thickness divided the formation there into four rather distinct are common throughout the unit. Where these lenses units with a total thickness of about 2,000 feet. The are thicker it is seen that they are channels cut in the lowest member is about 600 feet thick and composed argillite and are commonly graded. predominantly of light-colored carbonate-bearing On the north flank of Deer Lake Mountain, the upper siltite, quartzite, and argillite. The second member is part of the Wallace Formation is well exposed in large predominantly dolomite and is about 400 feet thick. roadcuts above the south shore of Deer Lake. The rock Member three is composed of about 300 feet of dark there is highly quartzitic and carbonate bearing. It is well-laminated argillite and siltite, and the fourth, identical with that in the lower part of the formation uppermost member consists mainly of distinctive red and was originally mapped by Miller (1969) as lower arkosic quartzite, about 700 feet thick. All but the low­ Wallace. Natural exposures are slightly better on the est member correlate well with similar divisions in the west flank than on the north. The carbonate-bearing· report area. 12 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

STRATIGRAPHY AND LITHOLOGY are rarely seen on a fresh surface. Most siltite beds are Although that part of the Belt Supergroup above the about %-lh inch in average thickness, but a few Wallace Formation in the report area is similar to the quartzitic beds are as much as 3 feet thick. Because Striped Peak Formation at other places, the differences of its thin-bedded character, the unit forms platy talus that exist almost justify using a new name here. For slopes below prominent outcrops. several reasons, however, not the least of which is the ApproXimately the upper 100 feet on Deer Lake confusion already existing in Belt nomenclature, the Mountain is thinly laminated deep-reddish-purple or rocks above ,the Wallace Formation in the report area maroon argillite and siltite with thinly interbedded are assigned to the Striped Peak Formation. Facies lensoidallayers of fine-grained quartzite. Interbedded changes that are large and rapid relative to facies near the top of the maroon zone are several platy­ changes in the lower Belt formations are recognized in weathering light-gray to pale-lavender siltite layers 10 the Striped Peak Formation in western Montana and feet thick. The Addy Quartzite rests unconformably northern Idaho. Most of the differences between the upon the maroon zone, indicating. that some of the for­ Striped Peak Formation in the report area and the mation has been removed by erosion here. On J umpo:ff formation in the Clark Fork quadrangle and the Coeur Joe Mountain, pre-Addy erosion has removed the entire d'Alene district can be explained by facies changes no maroon zone, and on the mountain south of Beitey greater than those described between localities of com­ Lake the upper contact is a fault. The thickest section parable separation at other places in Montana and of this member appears to be on Deer Lake Mountain, Idaho. where 900 feet of strata may be present. ' The formation is divided into four members, which Detrital mica is sparsely scattered through much of are designated by letters to avoid confusion with the the rock in both siltite and argillite layers. Large polyg­ numbered members in the Clark Fork quadrangle onal mud cracks and thin zones of mud-chip breccia (Harrison and Jobin, 1963, pl. 1). A composite section are common in all parts of the member except for the is illustrated here (middle column, fig. 7A) because no upper maroon zone. Small ripple marks are common complete and unfaulted section has been found in the but not abundant. Salt casts are common in the lower report area. Many of the members are fault. bounded member of the Striped Peak Formation at other places in much of the area, and the contact between members but were not found in the report area. a and b is a fault wherever found. Although many characteristics of this member are similar to those of the lower member in the Clark Fork

MEMBER a quadrangle and in the Coeur d'Alene district, three characteristics are notably different and make it diffi­ On J umpo:ff Joe Mountain and Deer Lake Mountain, cult to correlate member a with the equivalent part of the lowest member of the Striped Peak Formation is the. section at the other two localities. About 55 miles made up of thin- to medium-bedded siltite with inter­ east in the Clark Fork quadrangle and 75 miles south­ laminated argillite partings. Another but smaller fault­ east in the Coeur d'Alene district, the lower member is bounded section of siltite on the mountain south of generally red, pink, or green, or it is light gray tinted Beitey Lake is probably part of this member also. The with one of these three colors (Harrison and Jobin, siltite appears to conformably overlie the black well­ 1963, p. K17; and Hobbs and others, 1965, p. 45). The laminated argillite of the upper part of the Wallace red color appears to be an important characteristic of~ Formation on Deer Lake Mountain. Although the com­ the unit over a very large area. Except for the maroon plete transition from one unit to the other is not zone in the upper part of the member on Deer Lake exposed anywhere, a composite of the transition zone Mountain, the rocks of member a are darker and less appears to be about 50-100 feet thick. colorful than those making up the lower member to the About 70 or 80 percent of the lower member is east. In addition to a color difference, the lower mem­ medium-gray to olive-gray siltite. Except for about 100 ber in the Clark Fork quadrangle and in the Coeur feet of argillite near the top of the member, the other d'Alene district is characterized 'by small quartz and 20-30 percent of the unit is dark-gray to black argillite, calcite-filled vugs, and quartzitic layers which contain which occurs chiefly as thin bedding-plane partings and carbonate minerals. Although these features were care­ in beds less than 1 inch thick. Although they some­ fully searched for, only local traces of carbonate miner­ times tend toward pink or dull green, the siltite beds als could be found in member a. do not vary much in color, perhaps because they have been thermally metamorphosed in varying degree by MEMBER b nearby younger plutonic rocks. On a weathered surface, Member b crops out in a narrow discontinuous belt some siltite beds show faint internal laminations, which from Eagle Mountain south to the Loon Lake copper PRECAMBRIAN ROCKS 13 mine, but is well exposed only on Parker Mountain and The dolomite beds in this member are considerably the mountain south of Cottonwood Creek. The unit is purer as a whole than in any other Belt carbonate unit chiefly medium- to thick-bedded dolomite containing in the report area, but contain, even so, several percent many argilhiceous bedding-plane partings. Nowhere. in sand, silt, and argillaceous material, in addition to the the area is a complete section of the member exposed, silica already mentioned. On the west flank of Parker as the lower contact is everywhere faulted. Because of Mountain, the dolomite contains an anomalously large internal faulting, a reliable composite section can be amount of sand about 700-800 feet below the top of the constructed for only part of the member. From the member. The sandy zone is only about 100 feet thick available exposures, however, considerable lithologic and grades downward again to relatively pure carbon­ variety is obvious. ate beds with bedding-plane partings of argillite. On Quartzite Mountain and Parker Mountain the Calculated from outcrop width, the maximum thick­ contact with overlying member cis fairly well exposed ness of that part of the unit still preserved is about in places. Noticeable differences in the upper part of 1,750 feet. Although the least disturbed section was member b are found along strike in the 1% miles sepa­ chosen for the calculation, the figure may possibly be rating these two localities. On Parker Mountain the too large owing to undetected· faults. The preserved gradation down section from member c to b is from part of the member is probably not less than 1,000 feet gray argillite into purple argillite and subsequently into thick, however, which is considerably thicker than the purple or maroon dolomite. Locally, about 10-30 feet 400 feet reported by Harrison and Jobin (1963, p. K18) of light-gray dolomite separates the purple argillite for member 2 in the Clark Fork quadrangle. from the purple or maroon dolomite. Lower in member Because of stratigraphic uncertainties stemming b at this locality, the maroon pigmentation of the car­ from the absence of a complete section of the Striped bonate is increasingly pale and tends toward gray. Peak Formation throughout the area, the possibility On Quartzite Mountain, numerous thin beds of pale­ cannot be ruled out that the predominantly maroon green carbonate-bearing siltite in zones as much as 10 carbonate of member b is separated by member c from feet thick are interlayered with the dolomite. The same the gray and tan carbonate. If member c were overlain lithologies are found south of Parker Mountain, where by a maroon carbonate unit and underlain by a gray member b crops out about 1 mile north of Beitey Lake~ and tan carbonate unit, the stratigraphic section would Here, however, beds of maroon dolomite and carbonate­ resemble that shown in figure lB. Although correla­ bearing siltite are interbedded with gray dolomite and tions with other sections would not be impaired, and green carbonate-bearing siltite, and the contact with might even be improved, this interpretation would member cis a fault. necessitate changing several structural interpretations. From the south half of Parker Mountain to the As best as can be determined, however, all the relative­ southernmost exposures of member b, the upper and ly clean carbonate rock is restricted to a single unit lower contacts of the unit are both faults. On the moun­ and is overlain by member c. tain south of Parket· Mountain, most of the dolomite is medium gray and pale tan; maroon or green beds MEMBER c occur only on the south flank. The gray and tan beds Member c is better exposed than any other member probably represent ·the lowest part of the member of the Striped Peak Formation, and indeed better than exposed and are also found in the easternmost expo­ any unit in the area ·with the exception. of the Addy sures of the member, near the top of Parker Mountain. Quartzite. It is particularly well exposed on the south In general, the most intense maroon coloring appears and west slopes of Quartzite Mountain east of Chewe­ to be in the upper part of the member. The coloration lah (figs. 4, 9). The member is composed chiefly of decreases in intensity down section into the more medium- to dark-gray laminated argillite interlayered abundant tan and gray beds. About half of the unit is with lighter gray siltite in beds generally less than 1 pale tan or gray on a fresh surface, weathers tan or red inch thick. The argillite beds are well laminated, but brown, and forms a brick-red soil. in some parts of the unit the laminations are obscure The thick carbonate beds contain closely spaced except on a weathered surface. laminae of silica. Some of these films of silica are nearly Most of the member is grayish argillite. About 500 perpendicular to bedding and on weathered surfaces feet above the base is 250-300 feet of light-gray, pale­ commonly exhibit a boxwork structure like that yellow, and pale-green siltite and siltitic quartzite. described by Harrison and Jobin (1963, p. K18). In Small flakes of detrital mica are found all through the the few places where stromatolites have been found, rock of this zone but especially along bedding planes. the form of the fossil is commonly outlined and accen-. Bedding averages 2-6 inches in thickness. The only tuated by thin films of silica. other lithology that is notably different from the argil- 14 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

into black argillite beds. Cross-laminations are common but not obvious. Ripple marks are rare and confined THICK­ LITHOLOGY DESCRIPTION NESS, almost entirely to the siltite and siltitic quartzite zone. IN FEET Soft-sediment breccias are found throughout most of

Argillite and siltite, pale-gray­ the member (figs. 5, 6). They are apparently the result green, fai·ntly laminated ; of slumping or some other penecontemporaneous defor­ shows mudcracks; minor 290 amount of soft-sediment mation that occurred before lithification. Most of the brecc ia; contains pale-pink soft-sediment breccias on Quartzite Mountain are con­ seams of coarser silt as much as 1/ 8 inc h thick fined to 'individual beds and truncated at top and bot-

Argillite, dark - gray w ith light laminations; zones of unit 1 a few feet thick; well 290 laminated ; abundant mud­ cracks; abundant soft-sedi· ment brecc ia , but confined to individual beds

150 1 and 2 abov e mix ed in about equal proportions

215

70 Argi ll ite, black w ith quartzitic siltite. Slightly phyllitic

Siltite and siltitic quartzite, light · gray ; also yellow 250 and green ; contains detrital mic a. Be d s 2 - 6 inches thick , nonlam inated; mud­ c racks and ripple marks. Near 70 base, alternates with medium­ gray lam i nated argillite

Quartz itic siltite, light-gray 215 with purple tinge; inter­ bedd ed w ith blac k laminated argillite

Argillite and argillitic siltite, 150 dark-gray, well · lam ina ted ; abundant ; soft-sediment breccia 110 interval Argi llite, black and white; Member b paper-thin laminations; looks siltY 1810 TOTAL

FIGURE 4.-Generalized columnar section of member c of the Striped Peak Formation on Quartzite Mountain. lite is about 300 feet of pale-gray-green argillite and siltite in the uppermost part of the unit. The rock is characterized by thin pink lensoidal seams of coarser silt about one-eighth of an inch thick. Sedimentary structures are abundant in this mem- · FIGURE 5.-Argillite and breccia from member c of the Striped her. Mud cracks and small crenulated siltstone dikelets, Peak Formation. A, Laminated black argillite. Shows chan­ or filled desiccation cracks, occur throughout the mem­ neling, clastic dikes, and graded bedding. Lighter layers are ber. Graded bedding is well developed in the siltstone graded from coarse to fine silt. Collected near top of Eagle beds, especially in the upper 500-1,000 feet of section Mountain. B, Soft-sediment breccia showing disrupted bed­ on Eagle Mountain, but most hand specimens show ding and clastic dikes which presumably formed prior to lithification of the argillite. Collected near the top of Eagle only a gradation from light to dark gray because the Mountain about 100 feet away from specimens shown in A grain size is so small. Light-gray silt beds grade upward and about 20 feet stratigraphically above it. · PRECAMBRIAN ROCKS 15 tom by perfectly undisturbed beds, as if the disturbed Below this, the rocks are similar to those that make up zones acted as a decollement before lithification. On member c on the mountain north of Beitey Lake. Eagle Mountain many of the beds bounding the soft­ The 1,810-foot-thick section on Quartzite Mountain sediment breccias are disturbed also and may have appears to be the thickest in the map area. On the formed under slightly different conditions. The dis­ north flank of the mountain, the thickness may reach turbed beds there range in thickness from about 1 inch 2,000 feet where pre-Addy erosion did not cut so deep, to more than 5 feet. but the apparently thicker section could not be accu­ The two small hills south of the Loon Lake copper rately measured, as the rocks are very poorly exposed. mine provide the best exposures of the uppermost part On Eagle Mountain incomplete exposure and possible of member c and the transition into member d. The structural complications combine to prevent an accu­ c;:ontact is placed at the base of the lowest thick series rate estimate of the thickness of member c. The yellow­ of maroon siltite beds. Some maroon siltite beds occur and green-tinted siltite and siltitic quartzite zone found below this, but they are confined to zones less than 3 on Quartzite Mountain could not be identified on Eagle feet thick and are separated by thicker zones of dark­ Mountain, but may have been cut out along the fault gray siltite and laminated argillite. Most of the beds bounding the unit on the east. Even so, the width of in the upper 200 feet of member c contain detrital mica. Below the maroon rocks is a zone of finely lamina ted olive-gray siltitic argillite about 50-100 feet thick. These rocks appear to be highly susceptible to altera­ tion and weather out in large thin plates. Below the laminated rocks is several hundred feet of interbedded dark argillite and siltitic quartzite. Some zones of pure poorly laminated black argillite are as much as 15 feet thick, and some quartzite beds as much as 5 feet thick.

FIGURE 6.-Soft-sediment breccia in the McHale Slate, probable correlative of member c of the Striped Peak Formation. Loca­ tion is a few hundred feet west of the map a rea, about 4 miles north of Chewelah. Photograph illustrates how soft-sediment breccias are confined to distinct layers and are bounded by FIGURE 5.-Continued. undeformed laminated argillite. Compare with figure 5. 16 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

outcrop on the south flank of Eagle Mountain suggests region that Bennett studied, divided the Deer Trail that the member is much more than 1,000 feet thick. Group into the Togo Formation, Edna Dolomite, At least part of this section is stratigraphically higher McHale Slate, Stensgar Dolomite, and Buffalo Hump · than the highest part of the member on Quartzite Formation. Mountain. A composite thickness of the member on All the formations are extensively exposed in the both Quartzite and Eagle Mountains is probably 2,000- Huckleberry Mountain southwest of Chewelah. In 2,500 feet. If the uppermost parts of the member, such that area, the Deer Trail Group and younger rocks form as found on the mountains north and south of Beitey what has become known as the magnesite belt for its Lake, are added to this figure, the minimum thickness large deposits of magnesite. This region is referred of the composite section would be between 2,400 and to frequently in the following formation descriptions 2,900 feet. because the various units are much more extensively MEMBERd exposed there than in the report area. For a more com­ plete description .of the Deer Trail Group, the reader is The uppermost member of the Striped Peak Forma­ referred to Bennett (1941), Campbell and Loofbou­ tion is exposed only on the mountains north and south row (1962), Becraft and Weis (1963), and Campbell of Beitey Lake and on the two hills south of the Loon and Raup (1964). Lake copper mine. (See fig. 10.) It is perhaps the most The assignment of rocks in the report area to the easily identified of all the Striped Peak members Deer Trail Group is questionable beca~e the rocks. because of its deep-maroon to hematite-red color. The closely resemble the upper part of the Belt Supergroup. only divergence from this color is on the north flank Without exception, however, all the rocks so assigned of the hill south of Beitey Lake, where layers of black are overlain or intruded by the greenstone of the faintly laminated argillite and siltite in zones as much Huckleberry Formation. The greenstone has not been as 40 feet thick are interbedded with the maroon rock. found associated with the known Belt rocks east of the The chief rock type is siltite, but argillite is common main north-south belt of Cambrian Addy Quartzite. in thin layers and as bedding-plane partings. Thin beds Therefore, where considerable doubt exists, the pres­ of quartzite also are found throughout the unit, but ence of associated greenstone is used as a criterion in they are not as abundant as· in member 4 in the Clark assigning rocks to the Deer Trail Group. Fork quadrangle. Bedding thickness ranges from about Pronounced facies changes in the Deer Trail Group %-3 inches and averages about three-fourths of an over relatively short distances are well documented in inch. Internally some of the beds are faintly laminated. the magnesite belt by Campbell and Loofbourow (1.962, Because it is thin bedded, this unit almost always pl. 2 }- and Campbell and Raup ( 1964), and equally breaks into plates or chips. large changes, but on a regional scale, are found in the A few feet of pale-green siltitic quartzite occurs in upper Belt Supergroup (A. B. Griggs, oral commun., the transition between this and the underlying member 1969; Harrison and Campbell, 1963, p. 1424). There­ c. These beds are similar to those between members fore, slight differences between the Deer Trail units· 3 and 4 that Harrison and Jobin (1963, p. K19) in the report area and the type formations in the mag­ described in the Clark Fork quadrangle. nesite belt are less remarkable than would be similar The Cambrian Addy Quartzite unconformably over­ differences between two sections in the lower Belt lies member din the report area. The member is thick­ Supergroup. est, 500-800 feet, just southwest of Beitey Lake. All workers who have studied these rocks since Ripple marks, mud cracks, and salt casts are well Weaver have considered both the Deer Trail Group and developed and abundant throughout the member. Huckleberry Formation to be Precambrian in age. Cross-lamination, scour and fill, and graded beddings The Deer Trail Group is truncated by intrusive rocks are found locally and are relatively small scale features. north of Chewelah but continues on the other side of the batholith in the Metaline quadrangle with the same DEER TRAIL GROUP unvarying northeast trend that it has in the magnesite belt. Possible correlatives of the Deer Trail Group in The rocks assigned to the Deer Trail Group are the Metaline quadrangle have been called the Priest largely those that were originally called the Deer Trail River Group (Park and Cannon, 1943, p. 6), and the Argillite by Weaver (1920, p. 59). Bennett (1941, p. 7) same rocks in Canada have been called the Priest River elevated the Deer Trail Argillite to group status, and Terrane (Daly, 1912, p. 258). Becraft and Weis (1963, one of the carbonate zones in the section to formational p. 16) were the first to suggest a correlation between status. Campbell and Loofbourow (1962, p. F8), in the Priest River and Deer Trail Groups but were unable a more detailed report on approximately the same to match individual units. · PRECAMBRIAN ROCKS 17

TOGO(?) FORMATION several di~continuous quartzite zones in the Hunters Rock that may belong to the Togo Formation under.-. quadrangle, which covers most of the southern part of lies 01ily one small patch in the report area, about 2. the magnesite belt. In the report area quartzite and miles east of Valley. The rock is sheared dark-gray­ argillite zones were found only northeast of Chewelah green argillite which looks identical with that found in and could not be matched unit for unit with those in at least five other Precambrian formations in the area. the magnesite belt. It is assigned to the Togo Formation only because it The west boundary of the formation is a fault in this appears to underlie the Edna Dolomite. area, and so only a minimum thickness can be esti­ The lithology is similar to that making up part of mated. From outcrop width, the preserved section is the type Togo Formation in the magnesite belt to the calculated. to be 850 feet thick. Campbell and Loof­ west (Campbell and Loofbourow, 1962, p. F9). Camp­ bourow estimated that the thickness is between 1,500 bell and Loofbourow (1962, p. F8) named the unit for and 2,500 feet in most of the magnesite belt but could the exposures around the Togo mine about 15 miles not accurately measure it because of poor exposures. due west of Springdale. There, the formation consists McHALE· SLATE of several thousand feet of dark-gray to black lami­ nated argillite. The upper part of the formation con­ Black, gray, and green laminated argillite underlies sists of about 1,000 feet of relatively pure quartzite at about half a square mile just east of Valley and is con­ the latitude of Valley but thins to the north. Both sidered to be part of the McHale Slate. As with all units Campbell and Loofbourow (1962, p. F9) and Campbell of the Deer Trail Group in this area, the assignment is and Raup (1964) noted that the argillitic part of the not above question, but it is the best correlation based formation is carbonate bearing near the top. No accu­ upon available evidence. The argillite resembles the rate estimate of the thickness of the Togo Formation type McHale Slate more closely than it does any other has been made because of inability to detect structure formation in the Deer Trail Group and displays many in the. monotonous section of ru·gillite. Campbell and of the distinguishing characteristics of the formation. Loofbourow (1962, p. F9) estimated that the unit is The McHale Slate was named by Campbell and Loof­ at least 4,000 feet thick in the southern part of the a.rea, bourow (1962, p. Fll) for the exposures in McHale but it may be much thicker. Becraft and Weis (1963, p. Canyon, 4 miles west of Chewelah. There the unit is 7) estimated the thickness from outcrop width in the between 1,000 and 1,500 feet thick, as it is in most of same general area to be about 20,000 feet. the magnesite belt. The dominant lithology, which var­ ies little, is gray, green, and brown laminated argillite. EDNA DOLOMITE The McHale Slate contains numerous beds of soft-sedi­ ment breccia, which were not described by Campbell Light-gray to tan dolomite crops out on three small and Loofbourow. The breccia resembles that found in hills immediately west of the Togo(?) Formation in sec. member c of the Striped Peak Formation. The unit also 18, T. 31 N., R. 41 E., northeast of Valley. I tis identical contains numerous thin graded beds composed of silt­ in all respects with the type Edna Dolomite which was and clay-sized particles. named and described by Campbell and Loofbourow In the report area the rocks of this unit·are generally (1962, p. F9). The rock is medium- to thin-bedded phyllitic, although bedding is usually recognizable. The impure dolomite, but it has a massive appearance. It formation is highly faulted, and so no section is com­ weathers light tan and forms a bright-hematite-red plete. One mile east of Valley about 2,000 feet of argil­ soil. A completely fault-bounded discontinuous belt lite is preserved between two faults but is not well that extends about 4 miles north from Chewelah also exposed; it also may be faulted internally. This appar­ is considered to be a part of the Edna. There, the lithol­ ently is the thickest possible section of McHale Slate ogies are the same as those northeast of Valley, except in the map area. for a zone of vitreous quartzite about 300 feet thick in the lower part of the exposed section and several zones STENSGAR DOLOMITE of argillite as much as 50 feet thick interbedded with The Stensgar Dolomite crops out north of Valley and the dolomite. Only small partial sections are exposed southwest and north of Chewelah. The lowest contact in the report area, and so the stratigraphy cannot be of the unit is nowhere exposed. It is either concealed described in detail. by younger rocks or cut off by faulting. The upper con­ Campbell and Loofbourow (1962, p. FlO) reported tact on the hill north of Valley is gradational into the a zone of slate near the middle of the formation in the argillite and siltite of the overlying Buffalo Hump(?) magnesite belt and probably more than one zone of Formation. In that area, the dolomite is predominantly vitreous quartzite. Campbell and Raup (1964) noted light gray or light tan with interbeds of maroon dolo- 18 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

mite, argillite, or siltite. Bedding thickness ranges from of siltitic argillite is medium to thick bedded. The rock 1 inch to about 2 feet, and internal laminations or algal is primarily medium to dark gray, but has a faintly red­ structures are generally present in the thicker beds. dish cast locally. Vitreous quartzite beds are not pres­ Maroon argillaceous partings occur between some beds ent, and in this respect the rocks differ from those in but are not numerous. the magnesite belt. The rocks in the report area are Weaver (1920, p. 58) named the Stensgar Dolomite assigned tentatively to the Buffalo Hump Formation and included it in his Deer Trail Argillite. Bennett chiefly because they conformably overlie the carbonate (1941, p. 7) changed the name Deer Trail Argillite to unit containing maroon dolomite, which is considered Deer Trail Group and elevated the Stensgar Dolomite part of the Stensgar Dolomite. to a formation. The magnesite in the area is confined to On the hill north of Valley, the upper contact of the this formation, which is found almost the entire length unit appears to be faulted, but at least 1,000 feet of of the magnesite belt. Thickness ranges from 300 to section is preserved and fairly well exposed. This figure 1,200 feet in the magnesite belt. There it is chiefly a is probably an accurate estimate of the thickness of the fine-grained light-blue or pinkish-gray dolomite, which unit preserved in the area. is thin bedded except where extensively recrystallized (Campbell and Loofbourow, 1962, p. F13). Campbell DEER TRAIL GROUP, UNDIVIDED and Loofbourow (1962, F15) noted that a zone of red­ Rocks presumably.belonging to the Deer Trail Group dish dolomitic slate is commonly found near the top crop out on the hill north of Springdale and north of of the unit and that a similar zone about 100 feet thick that on the mountain west of Jumpoff Joe Lake. The was also found at the base in a measured section at the rock at both localities is not distinctive enough to assign north end of the belt. to any specific formation. The thickness of the formation in the report area Black laminated argillite and some thin-bedded silt­ cannot be accurately estimated, owing to poor expo­ ite make up the section on the mountain west of Jump­ sure. All the exposed sections are incomplete; 1 mile off Joe Lake. These rocks. most closely resemble the southwest of Chewelah about 500 feet of the unit is McHale Slate or parts of the Togo Formation. The preserved. North of Valley a partial section of the unit rocks north of Springdale are about the same, except is about 1,000 feet thick, as calculated from outcrop for a much higher proportion of siltite to argillite. · width, but is probably faulted internally.

BUFFALO HUMP(?) FORMATION RELATION BETWEEN THE BELT SUPERGROUP AND THE DEER TRAIL GROUP About 1,000 feet of argillite and siltite on the hill north of Valley is questionably assigned to the Buffalo From the preceding descriptions it is apparent that Hump Formation. The name Buffalo liump was given both the Belt Supergroup and Deer Trail Group are by Campbell and Loofbourow ( 1962, p. F17) to a thick thick Precambrian sections composed chiefly of fine­ section of argillite and quartzite that overlies the Stens­ grained clastic rocks which usually exhibit a very low gar Dolomite in the magnesite belt. They estimated grade of metamorphism. Part of the Belt Supergroup that as much as 3,000 feet of the unit is preserved near is strikingly similar to the Deer Trail Group and may the middle of the magnesite belt but found that it thins be an approximate time equivalent. There are, however, to the northeast and southwest because of erosion prior obvious differences between these two sections, and to deposition of the Huckleberry Formation. their proximity to one another necessitates either facies The formation consists chiefly of quartzite and argil­ changes or a large structural discontinuity. lite and exhibits striking facies changes in the magne­ The Belt Supergroup underlies many thousand site belt. Where it intersects ·the Hunters-Springdale square miles and has been studied by many geologists, road, the unit is about 25 percent quartzite and 75 a few of whom, such as Schroeder (1952, p. 19) and percent argillite. Only 5 miles along strike to the Ross (1963, p. 88), briefly considered the relation southwest, the proportion of quartzite is 85 percent between the Belt and Deer Trail. In all previous stud­ (Campbell and Raup, 1964, calculated from their map). ies, however, investigators were more handicapped than Much of the quartzite is confined to zones and is vitre­ us in attempting correlations, because the sections com­ ous, medium to fine grained, and light colored. The pared were relatively far removed and, as a result, they argillite is slaty and dark gray; much of it is not lami­ were unable to make unit-by-unit correlations. nated, unlike most of the argillite in the Deer Trail Schroeder (1952, p. 18-20) compared the rocks in Group (Campbell and Loofbourow, 1962, p. F18, F20). the ·Bead Lake area with both the Deer Trail Group In the report area, most of the rocks assigned to this in the magnesite belt and the Belt Supergroup in the unit are well laminated or thin bedded; about 100 feet Clark Fork district. He recognized differences between PRECAMBRIAN ROCKS 19 the two sections and correctly concluded that the rocks sulfides, which are characteristically found in the unit, in the Bead Lake area, which he named the Newport have not been described in the Togo Formation. Group, were more similar to the Belt Supergroup. The obvious lithologic dissimilarities betwe.en the Campbell and Loofbourow (1962, p. F20) noted that formations above these basal units may have tended the similarity in lithology and sedimentary structures to discourage previous attempts at unit-by-unit cor­ and the great thickness of the Deer Trail Group com­ relation. Neither the Edna Dolomite nor the Stensgar bined to make "the resemblance to the Belt Series Dolomite, both relativeiy pure compared with Belt [Supergroup] striking." Although they discussed the carbonates, resembles any of the 10,000-18,000 feet of problem more thoroughly than anyone had before, they Belt section between the Prichard and Striped Peak were unable to make unit-by-unit correlations. The cor­ Formations. The only other unit in the Belt Supergroup relation of the two sections was also discussed, briefly, that is lithologically comparable with the Togo Forma­ by Becraft and Weis (1963, p. 17). tion is the upper part of the Wallace. Formation, which We have drawn heavily on the valuable work by is probably only slightly more than 2,500 feet thick, these men and have combined it with our own data to thus considerably thinner than either the Prichard For­ make tentative unit-by-unit correlation between the mation or Togo Formation. Nevertheless, units over­ Deer Trail Group and part of the Belt Supergroup. lying the Togo and upper Wallace Formations cor­ The correlations proposed here are tentative, main1y respond well. If this correspondence is meaningful and . because of the composite nature of the Striped Peak the Togo Formation and the upper part of the Wallace section in the report area, but also because of major Formation are correlative, then the Togo Formation stratigraphic and structural differences between these may not be as thick as was estimated. Indeed, evidence two thick sections and the units overlying them. Figure of another sort suggests that the Togo Formation is 7 summarizes the possible correlations between the considerably less than 20,000 feet thick. Deer Trail Group of the magnesite belt and the Belt Almost all estimates of the thickness of the Togo Supergi"oup of the report area and also shows the rela­ Formation are based on the excellent work of Bennett tion of the Belt of the report area to the Striped Peak (1941), Campbell and Loofbourow (1962), Becraft Formation of the Clark Fork quadrangle. The Striped and Weis (1963), and Campbell and Raup (1964), Peak units of the report area that are shown in figure .who are chiefly responsible for the present understand­ 7 are pieced together as accurately as possible, but ing of the Deer Trail Group. Although each of these faults that interrupt the section may introduce errors investigators has provided additional refinements on in thickness aJ?.d even in the order of superposition. earlier work, any study of the Togo Formation in the Figure 8 shows the present spatial relations between magnesite belt is hampered by the incredibly poor expo­ the Deer Trail Group and the Belt Supergroup in the sure and a lack of any traceable marker units other than report area. the quartzite at the top. This combination makes it im­ In both the Belt Supergroup and Deer Trail Group, possible to systematically delineate structures in this the basal formation is considered at least twice as thick thick argillite unit. The result is well illustrated by the as any other formation in that section. The Togo For­ wealth of complex structures which have been mapped mation has been estimated to be as thick as 20,000 feet, in the rest of the Deer Trail Group (Campbell and Loof­ and the only Belt unit to which it could possibly be bourow, 1962, pl. 1; Campbell and Raup, 1964) and the compared on the basis of both thickness and lithology almost total absence of faults and folds shown in the is the Prichard Formation. Togo Formation, even where attitudes are locally Notably contrasting lithologic differences .make cor­ highly divergent. It would seem reasonable that faults relations between the Prichard and Togo Formations and folds are as numerous in the Togo Formation as in suspect, even if the Togo is considered to be as thick the rest of the Deer Trail Group and that the formation as the Prichard. Although both are made up of argillite, may therefore be thinner than previously estimated. If siltite, and quartzite, the relative amounts in the two the fault density in the rest of the Deer Trail Group is formations are considerably different. The Togo For­ superimposed on the Togo Formation, a thickness of mation is composed chiefly of dark-gray argillite with 3,000 feet could more than adequately account for the local, discontinuous carbonate zones and has a medium­ observed outcrop width. grained· quartzite zone as much as 1,000 feet thick at The Togo Formation and the upper part of the Wal­ the top. The Prichard Fqrmation has several distinct lace Formation are similar enough lithologically to be argillite and quartzite zones; all the quartzite is fine considered correlative. Both are thick sections made up grained, and most of it is 4,000 feet or more below the chiefly of laminated dark-gray argillite, both contain top of the unit (fig. 3). Both quartzite and siltite are carbonate-rich zones, and both are overlain by sections more abundant in the Prichard Formation, and iron containing a significant proportion of relatively coarse DEER TRAIL GROUP BELT SUPERGROUP IN BELT SUPERGROUP ~ RELIABILITY OF INDIVIDUAL LITHOLOGIC RELIABILITY OF INDIVIDUAL LITHOLOGIC IN THE MAGNESITE THE REPORT AREA IN THE CLARK 0 CORRELATIONS CORRELATIONS 2 BELT' (COMPOSITE SECTION) FORK QUADRANGLE

Addy Libby Formation

.,:,(. Good, but much coarser co c: grained at Clark Fork Cl) 0 0 0.. ·;; ::t: "0 co tz:j a.Cl) ...E Excellent, but requires ·;: 0 ~ large change in thickness Ci)U. tz:j t"'

t') > c: 0 f ~ ";i 0 co In part similar but re­ ... E 8 quires facies change :0 0 0 a.u. z ~ ~ t"' a. co 0.- > Excellent :J(ijs: ~ > ~ tz:j ~

Excellent ~tz:j ~ z C/.l > ~ C/.l ~ 0 Excellent ~z tz:j 0 0 ~

1-4~ tz:j .oo

C/.l~ ::t: 1-4z ~z

A DEER TRAIL GROUP BELT SUPERGROUP IN BELT SUPERGROUP RELIABILITY OF INDIVIDUAL LITHOLOGIC RELIABILITY OF INDIVIDUAL LITHOLOGIC IN THE MAGNESITE THE REPORT AREA IN THE CLARK CORRELATIONS CORRELAT-IONS BELT' (COMPOSITE SECTION) FORK QUADRANGLE 2

Huckleberry Libby Formation Formation

~cu c: cu 0 Highly suggestive litho­ CL -- logic similarity, but re. -om c» E Fair to poor- rapid facies a.~ quires major facies ·;:: 0 changes well documented change c;;u.. in this part of the section.

however I") c: -~":_-~;_:s~· om-~ .... E ~ ~ cu 0 ~u.. ~ Good c»CD Excellent, but requires a.u a.~ large change in thickness :>iii ~

Excellent "'tj

EXPLANATION ~ 0 > ~ Ill tJ:j Maroon or red Excellent clastic rocks ~ >z ~ ~ Dolomite 0 0 Fair to poor ~ ~ ~ '(f). FEET Sandy dolomite 4000

Argillite Excellent 3000 -~ ~ Siltite r:::l 2000 ~ 1 Campbell and Loofbourow (1962) 2 Harrison and Jobin (1963) Quartzite • Divided into five uniu in the Clark Fork quadrangle. Possibly up to 7,700 teet of the upper part is equivalent to the upper Wallace Formation in the report area • Does not compare exactly with the upper Wallace For­ 1000 iii mation in the Clark Fork quadrangle or the Coeur ff Alene Limestone district. It is more argillitic than in the Clark Fork quad­ rangle and more carbonate rich than in the Coeur ff Alene ~ district • Probably not as thick as has been reported previously ~ 0 Sandy limestone B FIGURE 7.-Possible correlations between the Deer Trail Group and the Belt Supergroup. A, A single carbonate unit assumed to. occur in the Striped Peak Formation of the report area. Compare with B; see text for discussion. B, Two separate carbonate units assumed to occur in the Striped Peak Forlna.tion of the report ....,.t..:l area. See text for discussion. 22 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

Pzu + pCv + pCdt PRECAMBRIAN ROCKS 23

EXPLANATION clastics and carbonate rock. Unfortunately, distinguish­ ing internal characteristics are generally absent in both units. Although only about 2,500-3,000 feet of the upper Wallace Formation is preserved in the report Tertiary and Mesozoic intrusive rocks area, the unfaulted thickness before erosion could have been somewhat greater. The quartzite at the top of the Togo Formation has Paleozoic rocks, undivided a counterpart in member a of the Striped Peak Forma­ tion, although the lithologic correlation is not as close as that between the other units correlated. The Togo Cam brian rocks quartzite is chiefly light-colored medium- to fine­ Mostly Addy Quartzite and Gypsy Quartzite, but includes Maitlen grained quartzite with a small amount of interbedded Phyllite and some Metaline Formation in places argillite.. Member a, however, is predominantly darker siltite interbedded with some argillite. The Togo quartzite contains ripple marks and mud cracks similar Monk Formation to those that are so abundant in member a, but the two • units differ from the others correlated in the two sec­ tions in that they are not look-alikes. However, the Togo quartzite exhibits major facies and thickness Huckleberry Formation, Leola Volcanics, and Shedroof Con­ glomerate changes within the magilesite belt. Equal or larger differences could exist between the magnesite belt and the report area, especially since the Precambrian sec­ tions of the two areas appear to have been juxtaposed Deer Trail Group and Priest River Group by a thrust fault and thus were probably widely sepa­ rated when the rocks were deposited. Member a does not closely resemble member 1 of the Belt Supergroup Clark Fork and Coeur d'Alene sections either, even "though both are siltites. Rocks similar to those in mem­ ber 1 thin from a thickness of over 1,500 feet south of Contact the Coeur d'Alene district ( Shenon and McConnel, 1939, p. 5) to about 600 feet in the Clark Fork quad­ Fault rangle (Harrison and Jobin, 1963, p. K17), but the Dashed wl1ere approximately located units are lithologically similar at both localities. If t t t t ...... J.l. ou?uuonn?••u member 1 varies so greatly in thickness from north to Fault separating Deer Trail Group and Belt Supergroup Dashed where approximately located; dotted and queried where south, its lithologic character may possibly change concealed or destroyed by younger rocks. Sawteeth on presumed slightly to the west, and member a and the Togo upper plate; lraclwred where fault is downdropped by younger faults quartzite may be reflections of this change. Because only part of member a is preserved in the report area, --+------ft-- it is also possible that the carbonate-bearing siltite, Normal Overturned which characterizes unit 1 to the east, was deposited Axis of anticline, showing direction of dip of limbs in the report area but because of faulting is no longer Dashed where approximately located exposed at the surface. A remarkably good unit-by-unit match is evident ---t--t -?- between the remainder of the Deer Trail Group and Axis of syncline, approximately located Queried where concealed the Striped Peak Formation. The Edna Dolomite is - - indistinguishable from the light-tan and gray carbonate Deer Trail Group Belt Supergroup-- that characterizes all but the upper part of member b. Generalized strike of strata whe~e reliable data are available The McHale Slate is almost identical with member c FIGURE B.-Generalized geologic map showing the distribution and even displays the same type of soft-sediment defor­ of the Belt Supergroup and the Deer Trail Group in and mation that characterizes the member. Differences in around the Chewelah-Loon Lake area. No postintrusive appearance result largely from the greater dynamic rocks shown. Geologic data modified from Huntting, Bennett, Livingston, and Moen (1961), Schroeder (1952), Park and metamorphism of the McHale Slate. Although no car­ Cannon (1943), Bennett (1941), Campbell and Loofbourow bonate unit of significant thickness which could corre­ (1962), and Campbell and Raup (1964). late with the Stensgar Dolomite is known to overlie 24 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

member c, the maroon argillite, siltite, and quartzite equally thick section of greenstone constitutes the of member d might be compared with the maroon argil­ Huckleberry Formation in the magnesite belt. The for­ . lite that bounds the Stensgar Dolomite and seems to mation rests unconformably on the Deer Trail Group thicken toward the northeast end of the magnesite belt. there. The two lithologies and their stratigraphic rela­ The Buffalo Hump Formation at the top of ·the Deer tion to the rocks above and below them were first Trail Group has no equivalent in the Belt section of the recognized by Jones (1928, p. 115), who referred to report area, as the Cambrian Addy Quartzite rests un­ them collectively as the greenstone phase of the Chew­ conformably on the eroded surfaces of member d and· elah Argillite. Weaver (1920, pl. 1) earlier differenti­ older rocks. The Buffalo Hump Formation may be ated small areas of greenstone locally but at most roughly the same age as parts of the Missoula Group places included both conglomerate and volcanic rock that A. B. Campbell ( 1960, p. 560) described in the St. in the Addy Quartzite. Bennett (1941), p. 8) divided Regis-Superior area to the east or as parts of the Libby the conglomerate and greenstone and described them Formation. Either of these correlations would imply in detail, but he assigned these rocks to the Lower considerable changes in facies and thickness. Within Cambrian. He recognized the major unconformity be­ the magnesite belt, the Buffalo Hump Formation tween the conglomerate and the underlying Deer Trail changes from predominantly argillite at the northeast Group and coined the names Huckleberry Conglomer­ end to predominantly quartzite at the southwest end ate and Huckleberry Greenstone. Campbell and Loof­ (Campbell and Loofbourow, 1962, pl. 2; Campbell and bourow (1962, p. F24) showed that the contact Raup, 1964); thus major facies changes do occur in this between the greenstone and Addy Quartzite is also an part of the section. unconformity and assigned the conglomerate and greenstone to the Precambrian. They gave these two METAMORPHIC ROCKS, UNDIVIDED units member rank in a formation they named the Deformed mica schist injected by leucocratic dikes Huckleberry Formation. That usage is followed in this and sills occurs in several small areas in sec. 29, T. 34 report; N., R. 41 E., and sec. 34, T. 35 N., R. 41 E. These The Huckleberry Formation in the report area. is made~-up-- entirely-of .the greenstone me'mber. highly metamorphosed rocks cannot be reliably as­ aimost. signed to any formation. In addition, they lie near the Conglomerate; ·whld11~dound oniy on the hili' southwest boundary between the Deer Trail Group and Belt of Chewelah, was not differentiated from the green­ Supergroup, and so they cannot even be assigned to one stone on the geologic map. The largest area underlain or the other. by the formation is about 8 miles due north of Chew­ Highly recrystallized quartz-mica schist, quartzite, elah. Here the greenstone is faulted against the Addy and amphibolite underlie a narrow strip about 9 miles Quartzite on the east and is unconformably overlain long between Nelson and Goddards Peaks along the by the Monk Formation on the west. It is not known east border of the report area. All the rock in this area whether the absence of the conglomerate at this local­ was derived from the Prichard Formation. It is mapped ity is due to faulting or to nondeposition, but the latter separately from the Prichard Formation because a is suspected. On their geologic map of the magnesite short distance east of the report area it is not possible belt, Campbell and Loofbourow (1962, pl. 1) showed to distinguish from which formation the metamorphic the formation thinning toward the southeast, east, and rocks were derived. The metamorphic rocks, which are northeast, and as the formation is thin on the hill i mile intruded by many large and small bodies of two-mica southwest of Chewelah, it may wedge out a few ·miles quartz monzonite, extend to the west side of Pend north of the town. Oreille Valley, 6 miles east of the report area. The The large area of greenstone north of Chewelah con­ extreme recrystallization of the rocks was caused by sists of flows: bre~cias, a~d a min~-r amount of light­ the numerous bodies of two-mica quartz monzonite. coiored tuff. Petrographically, much of it resembles The contact with the Prichard Formation cannot be volcanic breccias and aquagene tuffs in British Colum­ located precisely because a gradational zone as much bia that Carlisle described (1963, p. 57). Fine-grained as 1 mile wide, separates the units and because there dark-green basalt appears to be the most common rock are very few exposures east of the divide separating type in the greenstone. Although these rocl{s are mildly Nelson and Goddards Peaks. chloritized-an.-d· epidotized, the primary igneous tex­ tures are perfectly preserved in many places. Iri several WINDERMERE GROUP specimens, pyroxene crystals (augite Ny=l.686, 2V= 54 °) are completely unaltered, although almost all HUCKLEBERRY FORMATION plagioclase reflects a low-grade metamorphism in that A thick section· of conglomerate overlain ·by an it is much more sodic (Ans:!:2) than an unaltered basalt PRECAMBRIAN ROCKS 25 should be. The Na20 content of the rock, however, is and the conglomerate are cut by a well-developed cleav­ too low for a spilitic rock, although alkalies may have age which imparts a phyllitic character to the rock and been removed during metamorphism. · in most places masks any bedding or layering that may Analyses (table 1) of three samples considered to be have been present. representative of the basalt flows and an analysis of a Attitudes in the large greenstone area north of fine-grained tuff show that the rock is chemically a Chewelah are difficult to obtain because of the almost basalt but contains unusual amounts of a few compo­ complete lichen cover and the gradational character of nents. The total iron content is high, and the calcium the contacts between flows and breccia horizons. At low. The analyses are most like those of a tholeiitic the few locations where attitudes could· be determined basalt (Nockolds, 1954, p. 1030), except that the silica with relative certainty, the sequence appears to strike content averages 1 or 2 percent low. Normative quartz approximately north-south and to dip westward at low is calculated in three of the specimens because of the angles. In at least one place in the eastern part of the extremely low alkali content. outcrop area, the greenstone dips eastward; thus the Whereas almost no. planar structures, primary or great width of the outcrop may be due to a broad open secondary, are developed in the greenstone north of fold. Chewelah, southwest of the town both the greenstone Because of the difficulty in determining attitudes in the greenstone the thickness of this formation cannot be accurately estimated. The few attitudes available TABLE 1.-Chemical analyses and CIPW norms, in weight per­ cent, of basalt and one tuffaceous rock from the greenstone were used in drawing a cross section across the strike of the Huckleberry Formation of the large outcrop area north of Chewelah. If the cross. [Analysts: P. L. D. Elmore, S.D. Botts, Lowell Artis] section is correct, the greenstone must be at least 1,200 2 3 4 5 feet thick to account for the outcrop width. As the base Chemical analyses is not exposed, the unit could easily be much thicker. 8102 ...... 42.6 43.8 44.8 47.5 In several small areas 6 miles north-northeast of Ah03 .... -...... 12.7 12.0 13.2 13.4 FE!:!Os ...... 95 1.6 1.0 2.4 Chewelah and at several places north and east of Val­ FeO ...... 13.5 12.2 11.5 10.9 MgO ...... 7.3 7.1 6.3 7.5 ley, greenstone appears to intrude the rocks of the Deer CnO ...... 5.0 7.8 6.6 6.7 Na20 ...... 1.9 .95 2.8 2.1 Trail Group. Similar dikelike bodies of greenstone too K20 ...... 59 .28 .85 .9 H20-...... 14 .15 .18 .43 small to show on the map are found on the hill west of H20+ ...... 5.0 5.7 4.3 4.4 TI02 ...... 3.5 3.2 2.8 2.3 J umpoff Joe Lake. All these intrusive bodies are prob­ P20G ...... 72 .54 .45 .25 MnO ...... 20 .21 .21 .22 ably related to the extrusive rocks of the Huckleberry 002 ...... __§.&._ ...... !:..L __i:!_ .24 Total...... 100.60 99.83 99.79 99.24 Formation. Some caution must be exercised in assign­ Chemical analyses (calculated on H20- and C02-free basis) ing greenstone, whether intrusive or extrusive, to the 8102...... 47.9 48.8 49.5 50.4 48.7 Huckleberry Formation, for similar-appearing green­ AbOa...... 14.3 13.4 14.6 14.2 14.1 FE!:!Os...... 1.1 1.8 1.1 2.6 1.3 stone of Paleozoic age is present 17 miles southwest of FeO...... 15.2 13.6 12.7 11.6 13.8 MgO...... 8.2 7.9 7.0 8.0 7.7 Chewelah in the Hunters quadrangle (A. B. Campbell, CnO...... 5.6 8.7 7.3 7.1 7.2 Na20...... 2.1 1.1 3.1 2.2 2.1 written commun., 1965). K20...... 66 .31 .94 .96 .64 Ti02...... 3.9 3.6 3.1 2.4 3.5 In the Metaline quadrangle, about 40 miles northeast P20G...... 81 .60 .50 .27 .64 MnO ...... 22 ~ ~ ~ ~ of Chewelah, Park and Cannon (1943, p. 7-11) Total...... 99.99 100.04 100.07 99.96 99.91 described some 5,000 feet of conglomerate that uncon­ CIPW norms (calculated on H20- and C02-free basis) formably overlies argillite of the Priest River Group. Q...... 1.4 6.3 2.0 The conglomerate is in tum overlain by some 5,000 c...... 1.8 ...... 1:8 or...... 3.9 5.6 5.6 nb...... 18.1 9.0 26.2 18.8 feet of greenstone. They applied the names Shedroof an...... 22.6 30.8 23.1 20.0 wo ...... 3.5 4.1 3.2 Conglomerate and Leola Volcanics to these two units en...... 20.4 19.7 12.4 19.8 fs...... 20.9 18.0 12.6 15.6 and correlated them with the formation Daly (1912) fo...... 3.5 named Irene Conglomerate (later named the Toby Con­ fa ...... 2:6 3.9 mt...... 1.5 1.6 3.7 11...... 7.5 6.8 5.9 4.6 glomerate by Walker, 1926, p. 13) and Irene Volcanics ap...... !.:!.. ~ .....!.:!.. _._6 in Canada. Becraft and Weis (1963, p. 173) correlated Total...... 100.0 99.9 10Q.l 99.9 the conglomerate of the Huckleberry Formation with Percent normative an In plagioclase...... 55.7 77.2 47.0 58.8 59.6 the Shedroof and Toby Conglomerates, and the green­ 1. Fine-grained basalt, 700 ft E., 2,100 ft N. of 8W cor. sec. 26, T. 34 N., R. stone with the Leola and Irene Volcanics.· 40 E. 2. Fine-grained basalt, 1,050 ft E., 1,850 ft N. of 8W cor. sec. 26, T. 34 N., The conglomerate northeast and southwest of the R. 40 E. 3. Fine-grained basalt, 1,500 ft E., 1,550 ft N. of SW cor. sec. 26, T. 34 N., report area is fairly thick, and its near absence within R. 40 E. 4. Fine-grained tuff, 4,200 ft E., 1,700 ft N. of SW cor. sec. 26, T. 34 N., the area suggests either nondeposition due to a topo­ R. 40 E. 5. Average composition of the three basalts. graphic high or rapid thinning in a southeasterly direc- 26 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

tion. The basin in which the various conglomerate and sequence of units near the base of the formation in the volcanic units were deposited appears to have been Metaline quadrangle, except that the thicknesses of fairly extensive in a northeast-southwest direction the individual units differ from those in the report area. (Becraft and Weis, 1963, p. 16), but little is known They reported that conglomerate forms the base of the about its extent, if any, to the northwest or southeast~ Monk Formation north of the headwaters of Gypsy CI"eek and is overlain by 200-'-300 feet of limestone. MONK FORMATION South o£ the Gypsy Creek headwaters, however, the The Monk Formation is poorly exposed in the report conglomerate is missing, and presumably the same area, except on the mountain west of Cliff Ridge and for limestone rests on greenstone of the Irene Volcanics, a distance of about 3 or 4 miles to the southwest (out­ the Huckleberry greenstone equivalent. In and just side the quadrangle). The most distinguishing char­ west of the report area, clasts in the conglomerate are acteristic of the formation is probably its heterogeneity. pebbles and cobbles of greenstone from the Huckle­ Although predominantly slate and argillite, it contains berry Formation, an andesitic-looking rock not found numerous beds of dolomite, conglomerate, and quartz­ in the ·area, and fine-grained white quartzite. The ite. matrix is metamorphosed but ·appears to have been The Monk Formation was named by Daly (1912, p. carbonate-bearing argillite or siltstone that was sandy 148) for exposures of slate, phyllite, schist, and con­ in places. The slate overlying the dolomite west of Cliff glo·merate along Monk Creek in southeastern British Ridge is the most common lithology in the formation. Cr ,}umbia. He measured a section about 5,500 feet It is purple to maroon with thin pale-green beds which thick. Park and Cannon (1943, p. 11) estimated the range in thickness from about one-sixteenth inch to thickness of the formation to be about 3,800 feet in the about 2 feet. The purple beds are chiefly argillite, but Metaline quadrangle. There, it is mostly fine-grained the pale-green layers are composed of slightly coarser phyllite with numerous interbeds of carbonate rock, grained silt-sized material. Almost all the argillite is quartzite, and gri,t. finely laminated, although in much of it the lamina­ Rocks probably belonging to the Monk Formation tions are quite subtle. Both the purple and green are found in the hills bordering Bayley Creek, on the layers are carbonate bearing. Higher in the section, the mountain west of Cliff Ridge, and on the hill 1 mile slate contains progressively less carbonate, and in the southwest of Chewelah. The patchy occurrence of this upper half of the unit almost no carbonate-bearing unit is due chiefly to faulting, but the variation in thick­ rocks are found. ness also results from Precambrian erosion. The upper The small fault-bounded outcrops of this unit 4.5 contact is everywhere an unconformity with the Addy miles due north of Chewelah are made up of both the Quartzite. Some of the rocks, especially those east and lower dolomite and the purple slate. At this locality, southeast of Bayley Creek, may not be part of the however, the transition zone contains several tens of Monk Formation, but were so assigned because they feet of pisolitic sandy carbonate beds. The pisolitic are apparently underlain by the Huckleberry Forma­ rock is light yellow brown and forms beds as much as tion. Z. feet thick. It grades upward bed-by-bed into slaty On the mountain west of Cliff Ridge, dolomite of the maroon argillite. Monk Formation rests on greenstone of the Huckle­ The Monk Formation west of Bayley Creek and on berry Formation. The dolomite is pale yellow to pale the hill 1 mile southwest of Chewelah is made up, in gray and highly recrystallized, and it occurs in beds part, of lithologies not found in the more extensive sec­ as much as 5 feet thick. It is restricted to the lower 150 tion west of Cliff Ridge and could be part of the Stens­ feet of the unit within the area and grades upward into gar Dolomite. West of Bayley Creek, the Addy Quartz­ a section that consists predominantly of slate, some of ite rests on about 100 feet of cream-colored dolomite. it carbonate bearing. About 1.5 miles to the southwest, The dolomite is faulted over about 150 feet of purple along strike, the lower carbonate zone thickens and is to maroon argillitic dolomite and carbonate-bearing overlain by rocks displaying an upward gradation a:r;gillite, which includes a 20-foot-thick bed of conglom­ similar to that found to the north. From there, for erate and pebbly mudstone near the top. This pebbly about 2 miles farther southwest, beds of conglomerate mudstone and the carbonate rocks below it may be part and carbonate-bearing slaty argillite separate the car­ of either the Monk Formation or the Stensgar Dolo­ bonate rock and the greenstone. Although poor expo­ mite. sures do not permit detailed stratigraphic observations, The Monk Formation is overlain by the Lower Cam­ the conglomerate part of the unit appears to become brian Addy Quartzite on the hill southwest of Chew­ progressively thicker to the southwest. elah. There the Monk consists of about 400 feet of Park and Cannon ( 1943, p. 12) found a similar medium-gray slaty siltite and argillite ;;tnd contains PALEOZOIC ROCKS 27 beds of light-gray dolomite as much as 50 feet thick in the lower 150 feet. The slaty rock and dolomite overlie, or are faulted against, about 350 feet of highly sheared conglomerate provisionally assigned to the Huckle­ berry Formation. This rock strongly resembles the conglomerate member of the Huckleberry Formation in the magnesite belt, but is also similar to the conglom­ erate found in the Monk Formation southwest of Cliff Ridge. The conglomerate overlies the maroon argillite, carbonate-bearing pebbly mudstone, and dolomite be­ lieved to be part of the Stensgar Dolomite. PALEOZOIC ROCKS ADDY QUARTZITE The Addy Quartzite, first described and named by Weaver (1920, p. 61-63), is a thick fine- to medium­ grained vitreous quartzite. It crops out at many places in the west half of the report area and is one of the best stratigraphic markers in the region. This unit and its FIGURE 9.-The bold cliffs are dip slopes of Addy Quartzite probable equivalent to the north, the Gypsy Quartzite, which form the west side of Quartzite Mountain. Member c underlie much of northeastern Washington. of the Striped Peak Formation underlies the quartzite just The best exposures of the Addy Quartzite in the beyond the lip of the cliffs. The exposures formed by member report a:rea are due east of Chewelah on the west flank c in the low swale near the center of the mountain and those of Quartzite Mountain, where about 1,500 feet of the behind the most prominent cliff on the south flank of the mountain are the best of any Belt formation in the area. The unit forms spectacular, near-vertical cliffs (fig. 10) . steep slopes on the hill in the right background are underlain Eagle Mountain mark~? the northern limit of an by the Revett Formation. View east. approximately north-south discontinuous belt of Addy Quartzite which extends southward to just beyond Springdale. This belt is bounded on the west by a fault of variable dip and unknown displacement. The fault trace is covered by glacial and alluvial material for the entire length of the belt. A surface of low relief was eroded across rocks of the Belt Supergroup after they had been faulted and broadly folded. The Addy sediments deposited on this surface therefore rest unconformably on several of the Precambrian units. At most places in the area, however, the angular discordance is so slight that it cannot be reliably measured. On the west flanks of Eagle Moun­ tain and Quartzite Mountain, the Addy rests on mem­ ber c of the Striped Peak Formation, and at various places south of Quartzite Mountain, it rests on mem­ bers a, c, and d (figs. 9, 10). On the hill1 mile south­ west of Chewelah, and on the mountain 10 miles north of Chewelah, just west of the report area, the formation rests on the Monk Formation. Southwest of Valley it appears to have been deposited on the Buffalo Hump(?) FIGURE 10.-Contact between the Addy Quartzite and member Formation of the Deer Trail Group. d of the Striped Peak Formation is at the base of the bold The unconformity at the base of the Addy Quartz­ outcrops in the left center of the picture. The contact is well ite is well exposed on the west flank of Quartzite Moun­ exposed here, but the angular discordance between the two tain and on J umpoff Joe Mountain. At both localities, units is so small that it cannot be measured. The base of member d is near the break in slope at the right edge of the the basal100-300 feet is a distinctive purple quartzite. photograph (arrow) . Outcops are on the small hill immedi­ These beds are well stratified and generally are marked ately south of the Loon Lake copper mine. View north. 28 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON by a well-developed thin black striping which is easily 1 mile south of Springdale. There, a few hundred feet mistaken for bedding. From the basal beds the Addy of medium- to thick-bedded white quartzite is overlain grades upward through pink quartzite over a strati­ by the Metaline Formation. An interval about 100 feet graphic distance of about 100-200 feet into the white wide between argillaceous limestone of the lowest Met­ and light-gray quartzite which characterizes most of aline and quartzite of the highest Addy is covered, and the formation. At several places in the area, the purple so it is not known if there is any slate or argillite quartzite is separated from the underlying Precambrian between the two units, as there is in sections north of rocks by as much as 500 feet of white, gray, or pink the report area. quartzite and siltite (Reynolds, 1968, p. 11). The most complete section of Addy Quartzite in the The bulk of the Addy Quartzite is white to light-gray vicinity of the report area is just east of the town of vitreous quartzite, although some beds have a pale­ Addy (fig. 2), about 4 miles northwest of Chewelah. yellow or pale-pink cast. Small pink quartz grains are There, a north-dipping, apparently homoclinal section sparsely scattered throughout this rock and are a char­ of Addy Quartzite about 4,000 feet thick overlies acteristic of the formation. The beds range in thickness Huckleberry greenstone and is reasonably well exposed. from a few inches to over 20 feet but average about 3 The upper part of the quartzite contact appears to be feet. Although a poorly developed bedding-plane part­ faulted against the Huckleberry Formation. ing is usually present and gives the rock a massively Only two fossil localities are known in the formation. bed.ded appearance, bedding, as defined by differences One is located about 2,500 feet above the base of the in grain size or lithology, is not. obvious owing to the section east of Addy. Fossils from this locality were homogeneity of the formation. Real bedding in the identified by A. R. Palmer (written commun., 1964) lower purple quartzite, as contrasted to the black strip­ and include Nevadella addyensis (Okulitch), Ku'tor- . ing, is defined by thin partings of argillaceous material gina?, and Hyolithes sp. Palmer concluded that the or by beds of coarser grained quartz. Beds of purple trilobite indicates that the quartzite is very Early Cam­ quartzite average 1-2 feet in thickness, but massive brian. The other locality is about 1 mile to the south­ beds as much as 5 feet thick are common. west, on the west side of Addy. Among those who have Reynolds (1968, p. 13, 15) studied the lower part of examined fossils from this second locality, Okulitch the formation in detail, including the purple beds, and (1951, p. 405) seems to have made the most detailed found that the rocks consist of fine to medium sub­ study. He reported finding the following fauna: Miera­ rounded to well rounded moderately to well sorted mitra (Paterina) sp.1 Kutorgina cf. K. cingulata (Bil­ quartz grains. These observations also apply to the rest lings), Kutorgina sp., Rustella cf. R. edsoni Walcott," of the formation. Small lenses of pebble conglomerate HyolitheUus sp., Nevadia [==Nevadella] addyensis are common in the lower 200-400 feet ·and are Okulitch, olenellid fragments, and fucoids. Okulitch abundantly exposed on Quartzite Mountain. Pebbles concluded that "The fauna is undoubtedly Lower Cam­ of quartzite are the most common clasts. Pods of con­ brian in age; and the presence of the rare genus Nevadia glomerate and sedimentary breccia as much as 5 feet [==Nevadella}, with its very primitive characteristics, thick occur locally at the contact with the underlying suggests the lower portion of the Lower Cambrian." Precambrian rocks, on Deer Lake Mountain, and Lithologically, the fossil-bearing rock is not typical Quartzite Mountain. The clasts in these pods are com­ of the formation as a whole. It is thin- to medium­ posed of the immediately underlying material and are bedded fine-grained quartzite which is locally argilla­ probably locally derived. · ceous and contains detrital muscovite. Beds range in Smalllensoidal pockets with a friable appearance are thickness from 1 to 12 inches and are interbedded with another feature chal'acteristic of the Addy Quartzite as argillite bands as much as 4 inches thick. The argillite a whole but much more common in the lower 500 feet. is medium gray, black, and gray green. Some of it is Much of the cementing material in these pockets has laminated, but most is not. In the upper part of the apparently been leached out from between the grains, fossiliferous zone are a few beds of dolomitic siltite. but the grains are held together strongly by a small Above the dolomitic zone are more thick to massive amount of silica cement. Where numerous, the pockets quartzite beds typical of the formation. None of these impart a vuggy appearance to the rock. atypical lithologies have been recognized within the At least 50 feet of shale, some of it sandy, occurs report area. about 1,000 feet above the base of the unit on Quartzite The purple quartzite near the base of the Addy is Mountain. This shale is well stratified and light gray, one -oTflie- mos.fimportant stratigraphic·markers in the pale green, and pale maroon. It is overlain by more region.; "but -appears' lo" "have .been. largely ignored' by light-colored quartzite identical with that below. The pre~ous workers. Bennett (1941, p. 9) briefly described upper part of the formation is preserved only on the hill the formation, but did not mention the purple zone. PALEOZOIC ROCKS 29

Campbell and Loofbourow mentioned a zone in which with apparent conformity on the Addy Quartzite. the bedding planes are emphasized by thin purple According toR. G. Yates, who examined these rocks bands, but did not specify in what part of the section it with the authors in the field, the rocks closely resemble occurs. On a brief trip the authors found the purple the lower and middle units of the Metaline Formation zone near the base of the formation on Stensgiir-Mo\ln~ in the Deep Creek area, about 60 miles north of Spring­ tain, which is abo\it in the 'center~ of the m'agnesite 'belt. dale. There, the formation varies considerably from Park and Cannon ( 1943) did not report a purple zone place to place owing to both facies changes and large in the Metaline quadrangle, nor did Yates (1964) in lateral movements along faults; complete sections are the Deep Creek area. more than 5,000 feet thick and consist chiefly of lime­ Becraft and Weis (1963, p. 12) ·reported that the stone and dolomite (Yates, 1964). In the northeastern Addy Quartzite is about 3,900 feet thick in the Turtle part of the Hunters quadrangle, 15 miles northwest of Lake quadrangle, 30 miles southwest of Chewelah. Springdale, Campbell and Raup (1964) reported that They correlated the formation with the Gypsy Quartz-· as much as 8,000 feet of limestone and dolomite oyerlies ite in the Metaline quadrangle. Forty miles northeast the Addy Quartzite. They assigned these rocks to of Chewelah, in the Deep Creek area, about 2,900 feet Weaver's (1920, p. 66) Old Dominion Limestone. Simi­ of the Gypsy Quartzite conformably underlies the. larities in lithology and faunal assemblages; and rela­ Maitlen Phyllite (Yates, 1964), but the section is tionship to the Addy Quartzite, make at least part of faulted at the base. On Crowell-Sullivan Ridge near the the Old Dominion a most likely correlative with the type section of the Gypsy Quartzite in the Metaline Metaline Formation in the area. quadrangle, the unit is 8,500 feet thick (Park and Can­ In the Deep Creek area and in the Metaline quad­ non ( 1943, p. 13). About 7 miles west of that, however, rangle, the Metaline Formation is separated from the the section is only about 5,300 feet thick. Gypsy Quartzite (correlative with the Addy Quartzite) by the Maitlen Phyllite, which is over 5,000 feet thick CARBONATE ROCKS in both areas. In the Hunters quadrangle and the repo!1; . Paleozoic carbonate rocks crop out discontinuously area, the Old Dominion·· Limestone and the Metaline from Eagle Mountain to south of Springdale. Only the Formation niay ·rest directly on the Addy Quartzite. A Cambrian, Devonian, and Mississippian Systems have covered area about too feet wide separates the highest been identified, but other systems may have gone unrec­ outcrops of Addy Quartzite from the lowest Metaline ognized owing to the sparsity of fossils. Exposures in carbonate on the hill southeast of Springdale, and con­ the area are inadequate to construct a complete com­ ceivably that much phyllite could separate the two posite section, and part of the section may not be units. As no phyllite float was found in the covered exposed owing to faulting or erosion. Thick sections of area, the carbonate rock is assumed to rest directly on Cambrian and Ordovician strata not found.iii.tiie ·rirpo"ii­ quartzite. · area occur to the west. in the·_ Hunters· quadrangle The absence of _the_l\~~~tJen P.~yJlite !n ~he sout)).em (Campbeli and. Raup, '1964) and. to~ the-north. in.the part of Stevens County is apparently due to nondeposi­ Colville area (Bennett, pl. 6, in Mills, 1962), Deep tion rather. than fa~lting. A major fault. with large lat­ Creek area (Yates, 1964), and Metaline quadrangle eral transport separates sections containing the phyllite (Park and Cannon, 1943, p. 17-22). This distribution from those without it. The fault separating the Belt may be due to rapid and large-scale facies changes; Supergroup and Deer Trail Group may also divide the however, it is equally possible that these units were Paleozoic section of the report area from that typified never deposited in the report area. Most of the Paleo­ by the Paleozoic rocks in the Deep Creek area. zoic rocks here may have been deposited at a distant Southeast of Springdale the base of the Metaline location and brought in by large lateral movements. Formation consists of gray to blue-gray limestone. Because of the discontinuous and patchy occurrence Irregularly shaped yellow-brown-weathering argilla­ of the Paleozoic carbonate rocks that are preserved, the ceous seams are distributed throughout the rock in an stratigraphic relations of these rocks are not well estab­ interwoven network. The surface of the weathered rock lished. Mu~h C?f t]:te carbonate rock, especially between is typically grooved or fluted, resembling a rillenstein Loon Lake and Jumpo:ff Joe Lake, is exposed o_nly in like surface. The rock is thick 'to thin bedded and gener­ small, widely separated outcrops and had to be mapped ally fine grained. About 4,000 feet northeast of the as undivided. ..Paleozoic.· · · · · - Addy-Metaline contact, most of the lower part of the Metaline Formation is repeated by a fault. There, num­ METALINE FORMATION erous beds of black shale as much as 1 foot thick, which About 1.5 miles southeast of Springdale, carbonate are not exposed to the southwest, are·interlayered with rocks of the Middle Cambrian Metaline Formation rest the argillaceous limestone. Above the argillaceous lime- 30 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON stone thick- to thin-bedded tan to light-gray dolomite grains throughout. Bedding is well defined, ranging is interlayered with a small number of thin dark-gray from a few inches to 5 feet in thickness. The texture shale layers. varies from aphanitic to coarse grained. The partial sections are very poorly exposed and may Sedimentary structures are found throughout the be repeated more than once by faulting. The upper unit and along with the dark color serve to distinguish contact is a fault, and so the thickness of the Metaline the dolomite from all other carbonate units. Oolites are Formation in the area is not known. Because of poor common, usually confined to individual beds less than exposures and uncertain structure, the thickness of 1 foot thick but locally occurring as irregular patches. even the preserved part of the formation cannot be Dolomi~ic conglomerate is also found throughout the accurately estimated. The part immediately above the unit. It is composed of angular to slightly rounded Addy Quartzite and southwest of the fault that repeats clasts as much as 1 inch across, which are almost the section may be homoclinal. If this portion of the identical in appearance with the surrounding carbonate section is not faulted, it is about 2,000-2,500 feet thick. matrix. Randomly oriented curved chips of white Trilobites and brachiopods were collected in the coarse-grained dolomite scattered through most of the SW% sec. 35, T. 30 N., R. 40 E. A. R. Palmer unit may be recrystallized fossils. Some resemble the examined two collections and identified the trilobites curved cross section of a brachiopod shell, whereas as Peronopsis, Bathyuriscus, Olenoides, and undeter­ others are cylindrical and may be pelmatozoan remains. mined ptychopariods, and the brachiopods as Linnars­ The exposed portion of the unit is estimated to be sonia and Acrothele. between 600 and 700 feet thick. Palmer (written commun., 1968) stated that the col­ The lower contact is covered by surficial deposits lections contain undoubted Middle Cambrian fossils everywhere in the report area. Beneath the younger and also that "The * * * [fauna] * * * is most probably cover unit 1 must be faulted against either the Addy of late Middle Cambrian age, and could be the same Quartzite or Metaline Formation. An exploratory oil age as the earlier faunas from the Metaline Limestone well drilled by the Empire Exploration Co. in the NE% [Formation]. Olenoides is a long-ranging genus and sec. 30, T. 31 N., R. 41 E., penetrated carbonate rock, precise placement * * * within the Middle Cambrian most of it dolomite, for 2,192 feet. At 2,664 feet quartz­ on its own merits is not possible." ite was penetrated immediately below a fault zone, although it is difficult to ascertain from the description · DEVONIAN OR MISSISSIPPIAN CARBONATE ROCK of well cuttings if the drill was in unit 1 before hitting Some of the carbonate rocks in the Loon Lake quad­ quartzite. rangle that Miller (1969, p. 3) described as Mississip­ UNIT 2 pian(?) may include strata as old as Devonian and are The contact between units 1 and 2 is reasonably well here referred to as Devonian or Mississippian. Lime­ exposed on the hill in sec. 19, T. 32 N., R. 41 E. Where stone containing fossils overlies these rocks with no examined, it is marked by a rather abrupt change from apparent unconformity. The lower age limit is less pre­ dark-gray to almost white dolomite. Like unit 1, unit cisely based on Upper Devonian fossils found in isolated 2 is internally homogeneous. The beds of white dolo­ outcrops of carbonate rocks. The lower dolomite of the mite at the base are representative of the unit as a earlier report is divided into two units, and the over­ whole, except that large oolites or pisolites are found lying dolomite and calcareous slate are retained as a only in the lower 10-20 feet. Other than bedding and third. the oolites near the base, the unit contains no obvious UNITl sedimentary structures, perhaps because of recrystalli­ The lowest unit in this sequence is a dark-gray dolo­ zation. Although the units bounding it are predomi­ mite that crops out discontinuously from the south nantly fine-grained or aphanitic carbonate rock, unit 2 flank of Eagle Mountain to the mouth of Cottonwood is almost uniformly coarse grained. Creek. It also crops out in a small area along the Bur­ Bedding is less well defined than in unit 1. Beds lington Northern Railroad tracks 1.5 miles northeast range from a few inches to about 5 feet in thickness; the of Springdale. Although not very extensive, the best average is about 3 feet. The thickness of the beds gives exposures of the unit are on the hill in sec. 19, T. 32 the rock a massive appearance, but close examination N.,R.41E. of weathered surfaces shows that most of the thicker The dolomite is uniformly dark gray, except for a beds are thinly laminated internally. A. K. Armstrong few zones which are a mottled light gray. No limestone of the U.S. Geological Survey examined specimens col­ beds were found in the unit, and in fact, very little of lected from this unit and concluded that "Oolites and the rock is even slightly limy. In places the dolomite coated carbonate lithoclasts in a matrix of smaller appears to be argillaceous, and it contains sparse sand pieces of the same material suggest a shoaling environ- PALEOZOIC ROCKS 31 ment for part of the carbonate" (oral commun., 1967). MISSISSIPPIAN CARBONATE ROCK These shoaling rocks alternate bed by bed with others Limestone and dolomitic limestone containing Mis­ containing algal structures suggestive of an intertidal sissippian fossils are found on the hill 1 mile north of environment (Miller, 1969, p. 3). No fossils other than Springdale and on a smaller hill1.5 miles east of Valley. the nondefinitive algae have·been found in the unit. Most of the limestone is medium gray to blue gray. The The unit is 500-650 feet thick on the hill in sec. 19, lower part of the unit contains a few zones of tan to T. 32 N., R. 41 E. The faulted section that is well tan-gray dolomitic limestone, some of which are lami­ exposed along the Burlington Northern Railroad tracks nated. Bedding thickness ranges from less than 1 inch 2 miles northeast of Springdale is nearly horizontal and to more than 15 feet but averages about 2 or 3 feet. at least 550 feet thick. The rocks at this locality are Chert, in beds and nodules, is common but confined to very light gray rather than white. Because most of the definite zones. On the small hill near the center of sec. units in this area are fault bounded, the rocks assigned 27, T. 30 N., R. 40 E., about 400 feet of limestone in to unit 2 here could conceivably be a different part of which cherty rock alternates with noncherty is well the section. They have been assigned to unit 2 because exposed. The cherty zones are about 80 feet thick and they contain. oolites, especially near the base ' and are the zones without chert about half that. Most of the underlain by d~rk-gray dolomite which resem hies unit 1. chert beds are mily a few i.nches thick, but some are as UNIT 3 much as 10 inches. Slightly higher in the section to the northeast, some of the thicker beds appear to have been Overlying the white dolomite is an extremely distinc­ bioclastic limestone that has been locally replaced by tive unit which, unlike some of the carbonate units is chert. recognized with confidence wherever mapped. It is ~ot The grain size of the limestone on the hill north of in~e~ally homogeneous like the other two units butiS madeup ci11efiy·o·f' IIght:co1ore.cfdolomife;maroori slate Springdale varies from coarse to fine and to some degree and. an .gradations. between. 'tile. ·two .. The- base--of"-th~ may coarsen in the direction of the coarse-grained unit. is" predominantiY""iigh.t.:-gray--and cream-colored quartz monzonite. Where dolomitic, the rock tends to be slightly coarser than the limestone and ranges from dolomite and rests with apparent conformity on the medium to coarse grained. Approximately 500 feet white dolomite of unit 2. About 40 or 50 feet above the above the base of the preserv-ed section there is about base, several beds as much as 2 feet thick of pale-gray­ 10 feet of dark-gray to black aphanitic limestone which green argillaceous dolomite and maroon slate are inter­ serves locally as a marker within the unit. Both the very calated with the carbonate rock. The maroon beds and the color of this zone distinguish it become thicker and more numerous upward in the sec­ fine grain size from the other rocks in the unit. tion. Two to three hundred feet above the base is a zone of thin-bedded maroon slate about 100 feet thick. The . Neither the upper nor the lower contact is exposed ~n the area. Northeast of Springdale the upper contact maroo~ rock grades into pale-green argillite both above. IS a fault, and the lower part is concealed beneath surfi­ and helm~ the -ione~ These coior. changes-are-especialf:Y-· well displayed in the exposures northeast of Springdale. cial material. A thickness of 600-700 feet for the pre­ The slate is generally well laminated and contains thin served _part of the unit is calculated from outcrop width, seams of dolomite. It grades upward into light-gray and but this figure may be in error because of undetected cream-colored dolomite similar to that in the lower faults. part of the unit. Fossils are found throughout the unit but are abund­ In the section northeast of Springdale, predominantly ant in only a few: places. The tan to tan-gray beds in light-gray to white dolomite that strongly resembles the the lower part of the unit contain abundant fenestellid rocks of unit 2 is found 500-600 feet above the base bryozoans and a few solitary corals. The medium-gray of unit 3. The contact between this rock and the under­ to blue-gray beds in the same general part of the section lying cream-colored dolomite of unit 3 is not exposed contain abundant pelmatozoan debris and a few fenes­ but is thought to be a fault. If it is a fault, the preserved tellid bryozoans, but all are partly recrystallized. Near part of unit 3 is between 500 and 600 feet thick. the middle part of the exposed section northeast of Bedding thickness in the dolomite ranges from a few Springdale is a dolomitic limestone bed which contains inches to about 10 feet. Although the dolomite is cream abundant corals, brachiopods, gastropods, and pelma­ colored (described as tan or gray tan by Miller (1969, tozoan debris. A. K. Armstrong of the U.S. Geological p. 3)), the weathered color, which is almost pale orange, Survey identified the following fossils (written com­ is more characteristic of the unit. The texture is partly mun., 1967): s~c~haroidal, but some of the rock is sufficiently apha­ Coral: nitiC to resemble chert. Amplexizaphrentis sp. 32 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE·COUNTIES, WASHINGTON

Brachiopods: entire Paleozoic is so incompletely exposed and poorly Unispirifer sp. indet. understood. S pirifer sp. indet. PALEOZOIC CARBONATE ROCKS, UNDIVIDED Pseudosyrinx? sp. indet. Gastropod: Much of the Paleozoic carbonate rock is mapped as. Platyceras sp. undivided because of the lack of distinguishing charac­ Armstrong reported the following: teristics needed to either assign the rock to established The fossils are poorly preserved as fragments which have units or define a new unit. In some of the areas shown been replaced by coarse chalcedony within recrystallized as bedrock on the geologic map, the exposure consists dolomitic limestone. Acid etching has yielded a small fauna. of only a few widely separated small outcrops. Some of The corals are typical representatives of Mississippian the undivided carbonate rock undoubtedly belongs to Amplexizaphrentis. A number of fragments of a brachiopod units already described, but much of it definitely does with a high cardinal area, non-plicated sinus and appar­ not. Although lithologic descriptions of many indi­ ently devoid of a syrinx may possibly belong to the genus Pseudosyrinx. The original shell structure was not pre­ vidual outcrop areas would serve little purpose, some served and the fragmentary nature of the material makes localities should be mentioned because the rocks there positive generic identification impossible. Fragments of contain a few fossils. pedicle valves strongly indicate the presence of both the Jones (1929, p. 43) reported finding brachiopods in genus Spirifer and Unisperifer. The fauna suggests a Mississippian age. a 2-foot-thick bed of purplish-red limestone in theSE cor. sec. 22, T. 33 N., R. 40 E. These were identified The limestone on the hill 1.5 miles east of Valley by Branson (1931, p. 70) as Kutorgina cingulata (Bil­ presumably belongs to the same unit. A. J. Boucot lings) of Early Cambrian age. The limestone is enclosed (written commun., 1966) recovered the following in light- to dark-gray dolomite, cream colored in places. brachiopods and suggested that the rocks were prob­ Most of the carbonate in this small area of outcrop is ably Mississippian in age: highly brecciated dolomite-so brecciated that bedding Rhipidomella sp. is difficult to recognize. · Composita sp. These fossil beds were searched for without success. indet. spirifer (fine ribs) Exposures are good only locally, and stratigraphic rela­ indet. spirifer (coarse ribs) tions are not clear in this area. K utorgina cingulata Crurithyris sp. (Billings) is found in the upper part of the Addy indet. brachiopods Quartzite, and Jones and Barnes may have collected Gilbert Klapper (written commun., 1966) examined the fossils from a fault slice of that unit. The lithologic rocks from the same locality for conodonts. Samples of descriptions by Jones and Branson are ·not detailed a light-tan dolomitic limestone bed were collected about enough to test this hypothesis, although Branson 30feet stratigraphically below the gray limestone col­ (1931, p. 70) mentioned that "the limestone is reddish lected by Boucot and yielded the following: in color, dense, sandy, and bears thin lenses of impure Siphondella isosticha (Copper) argillite." Some of the fossiliferous beds in the Addy Pseudopolygnathus multistriata Mehl & Thomas Quartzite east of Addy would fit this description. Gnathodus sp. The rocks from which the fossils came may also be Klapper concluded that "The * * * [age of the rocks] part of the Old Dominion Limestone of Weaver ( 1920), * * * is definitely Mississippian. It contains about the which overlies the Addy Quartzite north of Colville same fauna as that known from the basal Banff Forma­ (Bennett, in Mills, 1962, pl. 6). Although_ quite fossil­ tion of Crowsnest Pass, Alberta * * *. The fauna is one iferous, the limestone there contains only Archaeocya­ zone higher than the fauna described from the basal tha, however. Lodgepole***". (See Klapper, 1966, p. 5.) R. H. B. Jones and J.P. Thompson collected Late Embysk (1954, p. 14) identified the following ostra- Devonian fossils from a rather nondistinctive medium­ codes from the same locality: gray limestone in the north-central part of sec. 19, T. Graphiodactylus tenuis 31 N., R. 41 E. The rock appears to be interbedded Jones ina craterigera with light-gray and cream-colored dolomite, but expo­ Cavellina a:ff. C. corelli sures are not good enough to detennine even local Except for t_h~s~ .~pa,r.s_e.,Qqt.G:rops ip. the report area, stratigraphic relationships. The limestone is punky in no Mississippian rocks have been reported in north­ places owing to the presence of intricate solution eastern Washington or northern Idaho. The significance cavities. In addition to the brachiopods, the rock con­ of this fact cannot be competently evaluated by work tains solitary corals, which are poorly preserved, and in and around the report area, however, because the pelmatozoan debris. MESOZOIC PLUTONIC ROCKS 33

J. T. Dutro and Helen Duncan of the U.S. Geologi­ Amphissites cf. A. centronotus cal Survey examined these fossils and identified the Amphissites cf. A. simplicissimus brachiopod Cyrtospirifer sp., which, they remarked, "is She reported that the fauna is Pennsylvanian in age, widely distributed in rocks of Late Devonian age in the but she did not indicate exactly where in those three United States and elsewhere" (writtencommun., 1958). sections the fossils were collected. Despite a concerted Another collection was made from a locality a few hun­ effort to recover the locality, no Pennsylvanian fossils dred yards farther north by R. G. Yates, J. C. Moore, were found. Unfortunately, four of the Paleozoic car­ and F. K. Miller. The collection was examined by C. W. bonate units in addition to the carbonate rocks mapped Merriam of the U.S. Geological Survey, who stated as undivided are found in sections 23, 26, and 27. that the collection "probably is Upper Devonian as sug-. gested by Tenticospirifer which resembles Tenticospiri­ MESOZOIC PLUTONIC ROCKS fer utahensis of the Devils Gate Limestone in Nevada Plutonic rock underlies a total of about 150 square and other species of this genus in the Late Devonian miles at the north and south ends of the report area of Iowa. This fauna may be of about the same age as and intrude the complexly faulted and folded Precam­ those from the Devonian examined by Dutro and Dun­ brian and Paleozoic rocks. Eight·distinct plutonic bod­ can" (written commun., 1965). ies, ranging in· composition from granodiorite to alkali­ In 1927, R. H. B. Jones and J.P. Thomson collected rich quartz monzonite, have been mapped. These plu­ fossils from a poorly exposed fine-grained medium- to tons are part of the Kaniksu and Colville-Loon Lake light-gray dolomitic limestone in the NW% sec. 23, T. batholiths, following the terminology of Yates, Becraft, 30 N., R. 40 E. These fossils were identified in 1958 Campbell, and Pearson ( 1966, p. 56). by J. T. Dutro and Helen Duncan as follows: Geochronologic work by Joan C. Engels shows that Large crinoid columnals, indet. the plutons represent at least three periods of intrusive Fenestella sp. activity, although they are not easily grouped on the Cystodictya sp. basis of mineralogical or physical characteristics. The Leptargonia cf. L. analoga (Phillips) plutons range in area from about 1 square mile to more productoid brachiopod, genus indet. than 60 square miles. Most are irregular in shape, and Tetracamera? sp. the configuration of only a few appears to have been camarotoechid brachiopod, indet. controlled by preexisting structures. spiriferoid brachiopod, indet. Some plutons are not named because they are too Syringothyris? sp. small to be of regional importance or are obviously an Dutro and Duncan concluded that "The association of early- or late-stage variant of a larger pluton. Four of Leptargonia cf. L. analoga (Phillips), Tetracamera? the plutons considered to be of regional importance sp., and Syringothyris? sp., together with productoid have been named. and spiriferoid brachiopods, indicates an Early Missis­ The rock classification used is shown in figure 11. sippian age. A reservation is placed on this assignment Approximately 150 specimens were stained using the only because the poor preservation of the fossils make method developed by Laniz, Stevens, . and Norman positive identifications difficult" (written commun., (1964) and were modally analyzed. The number of 1958). The unit from which these fossils were collected specimens stained from any one pluton is a function of may be the same as the Mississippian limestone, but the size of the pluton and the variation of mineral com­ lithologic differences are too great to map it as that. position within the pluton. The greatest number of Embysk (1954, p. 15) collected the following fossils modes were determined for the larger plutons and those from li~estone found in sees. 23, 26, and 27, T. ·30 N., that showed noticeable variations in mineral com­ R. 40E.: position. In most specimens for which accuracy was Rhabdammina sp. checked, a total of 1,000-1,500 points insured no more Ammobaculites? sp. than a 2 percent error in any constituent. Point counts Endothyra sp. were made with a glass plate imprinted with a grid of M illerella sp. points spaced 0.06 inches apart. The plate was placed Globovalvulina cf. G. bulloides upon a stained slab, and the number of points falling Trochammina sp. on each mineral was counted under a binocular micro­ Lophophyllidium cf. L. proliferum scope. Rhombopora nitidula Modes of some fine-grained rocks were obtained using Spirifer aff. S. rockymontanus stained thin sections and methods described by Chayes ? Squamularia transversa ( 1956). Almost all thin sections used for petrographic Small Composita descriptions were stained for potassium feldspar. All 34 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

Quartz

10 35 65 K-feldspar P 1. Flowery Trail Granodiorite K P 2. Starvation Flat Quartz Monzonite K Plagioclase Boxed points are border zone specimens Plutonic igneous rock classification used in this rer>ort (see text)

p 3. Phillips Lake Granodiorite K p 4. Leucocratic dikes K p 5. Two-mica quartz monzonite K

p 6. Muscovite quartz monzonite K p 7. Coarse-grained quartz monzonite K P 8. Silver Point Quartz Monzonite K Boxed point is composition of fine-grained groundmass (see text)

p 9. Granodiorite K p 10. Fine-grained quartz monzonite K p Average modes of all plutons K

FIGURE 11.-Modes of plutonic rocks in the Chewelah-Loon Lake area. Circle is the average for each pluton. Modes recalculated to 100 percent quartz, plagioclase, and potassium feldspar. mineral identifications are from hand specimen or FLOWERY TRAIL GRANODIORITE thin section; no X-ray work has been done. Plagioclase LOCATION, EXTENT, AND TOPOGRAPmC EXPRESSION compositions were determined by measuring refractive indices using oil-immersion methods. The refractive The Flowery Trail Granodiorite, named by Clark and indices of amphiboles were measured in the same man­ Miller (1~68, p. 3), is an elongate pluton about 10 ner. Although a small amount of overlap of indices was square miles in area, one of the few entirely within noted, each pluton contains a hornblende of different .the report area. The west end lies about 1 mile east of refractive indices and presumably different composition. Chewelah, and the long axis extends east-northeast Table 2 summarizes the mineralogy and texture of from there for about 7.5 miles. The width nowhere the major plutons in the report area. exceeds 2 miles. About 5 square miles is covered by TABLE 2.-Petrographic data on plutonic rocks

Extent within Horn­ report area Potassium blende: Name of pluton (sq mi) Texture Average mode Plagioclase feldspar Hornblende Biotite biotite Pyroxene Accessories Flower Trail 10 Medium to Plagioclase, 38; Zoned from Mieroelineand X=light tan, X=light tan, :::::100-:::::::2 AB cores in Epidote, sphene, Granodiorite. coarse grained. potassium A1122toAnu; ortho:elase ( T) • Y=dark Y=Z=dark hornblende apatite, zircon, hypidiomorphic­ feldspar, 20; some albite olive green, olive green; on]y; 2V=60°. magnetite, granular. quartz, 11; rims. Z=bluish amount is ilmenite ( T) , mafies, 31. green, highly sericite,clino­ ny=l.686, variable. zoisite, calcite, ZAC=l4°. tourmaline. pyrite. Starvation Flat 35 Coarse grained, Plagioclase, 37; Zoned from Orthoclase(?) X=light tan, X =tan, (avg) .7 As cores in Apatite, sphene, Quartz Monzonite. hypidiomorphic­ potassium An20 to AIU5. (microperthite); Y =olive Y=Z= hornblende magnetite, granular. feldspar, 24; microeline. green, brown; near contact zircon, allanite, quartz, 26; Z=light euhedral. with green­ epidote, tour­ mafies, 13. green, stone of maline, musco­ ny=l.670, Precambrian vite, chlorite. Z/\C= Huckleberry calcite, hematite. 17°-25°, Formation. 2¥=66°-69°. Phillips Lake 60 Coarse grained. Plagioclase, 46; Avg Microcline ------.Absent ------·----.X=grayish tan, -··-·--····----.Absent ·----··-······-·-.Apatite, zircon, Granodiorite. Very slightly potassium composition Y =Z= allanite, epidote, foliated. Micas feldspar, 11; AD%2; zoned blackish magnetite, are interstitial quartz, 28; from brown; mus- garnet, tour- to, and wrapped mafies, 15. Ants to Alla'J. covite:biotite maline. around feld­ (includes =0.3 avg. spar and quartz. muscovite) . ~ tz:j r:n Leucocratic dikes See text section Medium to Plagioclase, 29; Avg Microcline ·------····-.Absent ------X=tan, ---··-·····-----.Absent ------·----Zircon, apatite,. 0 (associated with "Phillips Lake fine grained, potassium composition Y=Z= · garnet. N the Phillips Lake Granodiorite hypidiomorphic­ feldspar, 28; AD7. blackish 0 Granodiorite). (and associated to xenomorphic­ quartz, 33;. brown; mus­ 1-4 dikes)." granular. muscovite, 10. covite: biotite 0 (includes =2.1 avg. 'i:j muscovite). Two-mica quartz 2 Coarse grained, Plagioclase, 38; Avg Microcline ...... Absent ------X=golden tan, -----·-······-----.Absent ------Zircon, apatite, 8 monzonite. hypidiomorphic­ potassium composition Y=Z= opaque 1-:3 granular. feldspar 21; Ants. reddish minerals. 0 quartz, 30; brown; mus­ z mafies, 11. covite: biotite 1-4 (includes =0.3 avg. 0 muscovite). Coarse-grained 15 Very coarse Plagioclase, 33; Avg Perthite and Absent ------··------X=light tan, ---·-···········--·.Absent ...... Sphene, apatite, ~ quartz monzonite. grained, potassium composition microperthite; Y=Z= · zircon, 0 porphyritic ------·---- feldspar, 29; An2o. also occurs as brown; much magnetite. ~ quartz, 31; thin rims is chloritized. r:n biotite, 7. bordering quartz and plagioclase. Muscovite quartz 2 Coarse grained, Plagioclase, 34; Avg Microcline ...... Absent ------.Absent ------···········-----··-----····.Absent ------·------Garnet, magnetite, monzonite. hypidiomorphic­ potassium composition apatite, zircon, granular. Locally feldspar, 33; Ana. limonite. has texture quartz, 27; that may be muscovite, 6. protoelastic. Silver Point 30 Medium to Plagioclase, 40; Avg Perthite and X=light tan, X=tan, .9 Absent ------·-····-----SPhene, apatite, Quartz Monzonite. coarse grained, potassium composition mi,croperthite. Y=olive Y=Z=dark magnetite, zir­ porphyritic. feldspar, 25; An2o: zoned green, brown. con, allanite, quartz, 18; from Z=medium epidote, musco­ mafies, 17. An2o to Ana5. green, vite, chlorite. ny=l.662, Z/\C=17°. Granodiorite. %-1 Coarse grained, Plagioclase, 40; Zoned from Orthoclase(?) .•...... X=light tan, X=tan, 1.1 As cores in Sphene, apatite, hypidiomorphic­ potassium An2o to An3ll. Y =olive Y=Z=dark hornblende zircon, magne­ granular. feldspar, 22; green, brown. only. tite. quartz, 10; Z=medium mafies, 28. green, ny=1.663, Z/\C=13°. Fine-grained 1¥.! Medium to Plagioclase, 38; Avg Orthoclase(?) ...... X=pale tan, X=tan, 1.0 As cores in Sphene, apatite, quartz monzonite. fine grained, potassium composition Y=pale Y=Z=dark hornblende zircon, magne­ hypidiomorphie­ feldspar, 32; An20o olive green, brown. only. tite; allanite. granular. quartz, 12; Z=pale ~ mafies, 18. green, en ny=l.651, Z/\C=22°. 36 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON glacial and alluvial material. The Flowery Trail road effects are found is on the northeast flank of Quartzite traverses the length of the pluton and in the eastern Mountain, about 1 mile south of the pluton. There, part provides excellent roadcut exposures. carbonate-bearing rocks of the upper Wallace Forma­ A major canyon is incised along the length of the tion have been recrystallized to calc-silicate hornfels pluton and owes its presence, at least in part, to the of the low albite-epidote-hornfels facies of Turner and nonresistant nature of the rock. Natural outcrops, Verhoogen (1960, p. 511). North of the pluton, the where present, are usually quite weathered. The best only place where the metamorphic effects can be dis­ exposures are found where the granodiorite is in con­ tinguished from those of the Phillips Lake Granodiorite tact with more resistant metamorphic rocks. is on the south flank of Eagle Mountain. Highly recrys­ tallized rocks near the contact of the Flowery Trail INTERNAL FEATURES Granodiorite can be traced northward into relatively The Flowery Trail Granodiorite is an even-grained unmetamorphosed -rocks near the summit of Eagle hornblende-biotite granodiorite. Although relatively Mountain. Low on the north flank, the metamorphic small, the pluton shows the largest compositional var­ recrystallization again becomes apparent and increases iation of any in the report area. Modal analyses indi­ northward toward the Phillips Lake body. cate compositions ranging from granodiorite, through On the ·north side of the pluton, mineral assemblages· quartz monzonite, to monzonite. characteristic of the hornblende-hornfels facies of Tur­ In contrast to normal zonation in a plutonic'body, the ner and Verhoogen (1960, p. 515) extend to within a most quartz-rich spechnens are found around the mar­ few hundred feet of the contact. Assemblages include gins of this pluton. The quartz-rich rock at the margins, diopside-quartz-calcite-microcline and quartz-musco­ however, contains about 15 volume percent more mafic vite-biotite-plagioclase-andalusite. More than a few minerals than the rock in the center. This variation in hundred feet from the contact, host rocks at most places mafic minerals may reflect primary mineralogical zon­ are completely recrystallized, often with the formation ing, but the high quartz content of the border rock is of large porphyroblasts, but with the formation of min­ probably due to assimilation of. the host rocks-quartz­ erals characteristic of the albite-epidote hornfels facies. rich metasedimentary rocks of the Belt Supergroup. Common assemblages include quartz-albite-muscovite­ Some modes of rocks from the interior of the pluton do chlorite-clinozosite-green biotite, quartz-muscovite­ not indicate any zonal distribution of minerals. tourmaline, and quartz-albite-muscovite-actinolite­ If the zoning suggested by most of the modes is real, clinozoisite. it is too complex to be outlined by the few samples About half a mile east of the pluton, in the SW cor. whose modes have been determined. Unfortunately, in­ sec. 32, T. 33 N., R. 42 E., kyanite-bearing· schist is complete exposure due to heavy forest cover and glacial found in the upper part of the Prichard Formation. deposits precludes sampling at the density needed to Although this locality is not far from the Phillips Lake reliably determine whether the zonation is real or Granodiorite, the kyanite probably crystallized owing apparent. to the intrusion of the Flowery Trail Granodiorite. The latter is considerably closer, and when it was intruded CONTACT RELATIONS the amount of cover was probably thicker than when Most of the contact is concealed by glacial and allu­ the Phillips Lake body was intruded. Abundant andalu­ vial material or heavy forest cover. Even where the site schist is found on the east and northeast flanks of granodiorite is shown on the map, outcrops are some­ Goddards Peak in rocks of about the same chemical what patchy. The best exposures of the contact are in composition as those forming the kyanite schist, but the west half of the pluton in the vicinity of the Jay these rocks are closer to the Phillips Lake Granodiorite Gould mine, the Juno Echo mine, and the Blue Star and were probably crystallized by that pluton. mine. At these three localities, which are on both sides of the pluton, the contact with the metamorphic rocks PETROLOGY appears to be steeply dipping and is very irregular on Most of the Flowery Trail Granodiorite is medium to a small scale. Abrupt changes in direction every 10-100 fine grained. Locally the border rock is slightly foliate, feet are complicated by abundant, irregularly oriented but most of it is structureless. Inclusions are common granodiorite dikes and sills which intrude the host rock. though not abundant. Mafic minerals are abnormally The metamorphic aureole surrounding the Flowery rich in irregular patches as much as several hundred Trail Granodiorite varies in width. Along most of the square feet, and these contrast strikingly with the more south border of the pluton, noticeable effects appar­ leucocratic surrounding rock. If alpine exposures were ently do not extend more than half a mile into the host available, the rock would appear mottled on a large rocks. One of the outermost areas where discernible scale, with the light and dark areas blending over a MESOZOIC PLUTONIC ROCKS 37 distance of a few feet or less. Its high color index dis­ Minerals of the epidote group are the most common tinguishes the Flowery Trail Granodiorite from all alteration products in the granodiorite. Both epidote other plutonic rocks in the area except for a granodio­ and clinozoisite are present; highly birefringent epidote rite south of Springdale. ( ,-.0.35) is the most common. Epidote occurs as clus­ Plagioclase, the most abundant constituent of this ters of small anhedral to euhedral crystals around rock, occurs as subhedral to euhedral crystals that are hornblende and biotite and within plagioclase crystals. both normally and reversely zoned. Most of it is oligo­ Some are slightly pleochroic: Z'==very light green, clase; some rims are calcic albite, and some cores inter­ X'==colorless. Pyrite, sericite or muscovite, tourmaline, mediate andesine. In most sections the calcic cores are and carbonate minerals are the only other secondary obvious owing to large concentrations of small euhedral minerals in the granodiorite, and all but the sericite or clinozoisite crystals, calcite, and muscovite or sericite. muscovite are rare. These minerals, especially the clinozoisite and calcite, Changes in the characteristics of some minerals, evi­ indicate that the plagioclase cores were probably more dent only under the microscope, suggest a mild calcic before alteration and recrystallization. metamorphism that increases in an easterly direction. The potassium feldspar is microcline, although some Highly discordant apparent potassium-argon ages on untwinned crystals may be orthoclase. All crystals are hornblende-biotite pairs support this conclusion. (See anhedral and appear to have filled spaces between other section "Potassium-Argon· Ages of the Plutonic minerals. Where alteration of the granodiorite is obvi­ Rocks.") In thin section, plagioclase crystals are free ous, especially near the Jay Gould mine, only the potas­ of the cloudy haze of late-stage alteration products sium feldspar has been notably affected. There, the almost always found in normal plutonic plagioclase. In mineral appears to have been preferentially removed their place, scattered internally through the plagioclase, from the rock during mineralization. This relationship are anomalous-looking small euhedral crystals of clino­ is found only in the alteration zones around noticeably zoisite, epidote, and muscovite which were probably mineralized areas. formed by recrystallization of this late-stage alteration Clear anhedral quartz is present as small interstitial haze. Although metamorphism did not proceed far grains and less commonly as rims around plagioclase. enough to erase zoning in the crystals, in plane­ Some grains contain hairlike crystals of what may be polarized light they look like the clear qnaltered plagio­ rutile, and some have undulatory extinction. Most clase found in metamorphic rocks. These relations are grains are 0.04 inch or less across and are difficult to see most noticeable at the east end of the pluton, suggest­ in hand specimen. Because of this and the high mafic ing the source of the metamorphism was in that mineral content, the rock is easily mistaken for a diorite direction. in the field. Pleochroism and refractive indices of hornblende in Hornblende, the most abundant mafic mineral, is rock from the eastern part of the pluton are markedly euhedral to subhedral, averages about 0.08 inch in different from those in rock from the western part: length, and is easily identified in all hand specimens. Some crystals are corroded and embayed by biotite, X Y Z ny both around the edges and in the interior. Rounded West...... Tan ...... Olive green...... Blue green...... 1.679 cores of monoclinic pyroxene (2V ,_,60°) occupy the East...... Pale tan...... Olive green...... Green...... 1.691 centers of a few hornblende crystals. This is the only Eskola (1952, p. 166), Shido (1958, p. 171), and known occurrence of pyroxene in the Flowery Trail Shido and Myashiro ( 1959, p. 86) reported a change in Granodiorite. pleochroism of hornblende from blue green to green Biotite is present in all thin sections, but is always with advancing metamorphism. The changes they re­ less than half as abundant as hornblende. Crystals are ported were in regionally metamorphosed rocks, but the usually subhedral and closely associated with the horn­ comparison may be valid. blende. The metamorphism could have been caused by any , Sphene is by far the most abundant accessory min­ of several younger bodies to the east. The younger Phil.;. eral and makes up as much as half a percent of some ljps Lake Granodiorite and leucocratic dikes lie to the specimens. Crystals range in size from 0.004 to about north and east but may not be the sole cause of meta­ 0.12 inch. The characteristic wedge-shaped crystals are morphism, as the former shows some of the same meta­ visible in all hand specimens. Apatite is the only other morphic features found in the Flowery Trail Granodio­ abundant accessory mineral in the Flowery Trail rite. Younger intrusives are found about 10 miles south­ Granodiorite, although it is not as abundant as sphene. east of the Flowery Trail Granodiorite. Although these Zircon, magnetite, and ilmenite ( ?) are ubiquitous but intrusive bodies are probably not close enough to have not abundant. affected the Flowery Trail, discordances in potassium- 38 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

argon dates suggest that other younger plutons may be and highly metamorphosed sedimentary rocks are ob­ buried at shallow depth just east of the report area. scured because leucocratic quartz monzonite dikes and sills have intruded all three rock types. The configura­ EMPLACEMENT tion of the contact. between the Starvation Flat Quartz The location and shape of the Flowery Trail Grano­ Monzonite and the metamorphic rocks suggests a near­ diorite appear to have been controlled by two or more ·vertical relation. At localities far distanct from the east-northeast-trending high-angle faults. Rocks and quartz monzonite, the leucocratic dikes are closely preexisting structures which strike towards the pluton associated with the granodiorite and genetically related are bent parallel to it near the contact and suggest that to it. Since the quartz monzonite is intruded by the the emplacement was, at least in part, forceful. Abun­ dikes, it must be older than the granodiorite and pre­ dant dikes and xenoliths and apparent contamination sumably is intruded by it. by country rock near the contact, however, hint that South from Blacktail Mountain the inferred contact stoping and assimilation were at least as important as between the Starvation Flat Quartz Monzonite and the forceful intrusion. Phillips Lake Granodiorite is buried under glacial de­ bris, but appears to be essentially straight. A contact STARVATION FLAT QUARTZ MONZONITE this long and straight could be a fault, or it could be controlled by a preexisting fault. One well-located fault LOCATION, EXTENT, AND TOPOGRAPIDC EXPRESSION and two inferred faults are alined with the contact from the south and project toward it. Mylonite and cata­ Starvation Flat, located in the northwest corner of clasite are developed for about 3 miles to the northeast the report area, is underlain almost entirely by a tex­ and southwest along the contact from the point where turally and mineralogically uniform quartz monzonite. it intersects Little Bear Creek. The rock was named by Clark and Miller ( 1968, p. 3) in Along the south border of the pluton, the quartz the preliminary report on the Chewelah Mountain monzonite intrudes the Huckleberry Formation. How­ quadrangle, the north half of the report area. The ever, the contact is almost entirely covered by glacial pluton occupies about 20-25 square miles within ~n and alluvial sedmients, and its attitude is not known. area extending on the south from the abandoned Chfi Just outside the west boundary of the area, the contact Ridge Lookout to the north and west boundaries of the swings south and roughly parallels the boundary of the quadrangle. Gla<;:ial debris covers much of the north­ area for 2 miles. Here the contact is fairly well exposed west corner of the report area. The east side of the and quite irregular in configuration. The outcrop pat­ pluton terminates at a high-angle fault of probably tern and narrow width of the metamorphic aureole sug­ large displacement, which places it against the Phillips gest that the contact is generally steeply dipping. Lake Granodiorite. The quartz monzonite is found Pelitic, basic, and carbonate-bearing quartz-feld­ in many of the roadcuts along State Highway 294 north spathic rocks of the Monk Formation underlie theW~ and west of the quadrangle and thus extends well out­ sec. 27, T. 34 N., R. 40 E., at the west margin of the side the report area. Seventeen miles northeast of Calis­ map area. About 400 feet from the quartz monzonite pell Peak, quartz monzonite that is texturally, min­ contact, the. carbonate-bearing rocks are recrystallized eralogically, and modally identical with the Starvation to plagioclase-diopside-grossularite-quartz hornfels, Flat Quartz Monzonite crops out in large roadcuts and the basic rocks to plagioclase-diopside-hyper­ along State Highway 31, just north of Lost Creek. sthene-quartz hornfels characterizing the pyroxene The pluton forms a topographic low in the general hornfels facies of Turner and Verhoogen (1960); in the landscape and attains significant relief only near con­ interval from approximately 400 to 1,500 feet, actin­ tacts with the more resistant. metamorphic rocks. The olite replaces the hypersthene resulting in an assemb­ rather subdued topography around Starvation Flat is lage characteristic of the hornblende hornfels facies. typical of that underlain by the Starvation Flat Quartz Beyond about 1,500 feet metamorphic effects diminish Monzonite. Although not mapped in detail, at least abruptly. part of the contact between the quartz monzonite and In contrast to the Monk Formation, noticeable re­ the metamorphic rocks to the southwest can be inferred crystallization effects in the Huckleberry Formation from topographic differences. extend only a few hundred feet from the quartz monzo­ nite. At greater distances changes in the greenstone CONTACT RELATIONS are microscopic. The metamorphic effects of the Star­ On Blacktail Mountain, near the north border of the vation Flat Quartz Monzonite and the Phillips Lake area, intrusive relationships between the Starvation Granodiorite on the host rocks between them cannot Flat Quartz Monzonite, the Phillips Lake Granodiorite, be differentiated by source. MESOZOIC PLUTONIC ROCKS 39

PETROLOGY tent optical properties throughout the pluton. They are The Starvation Flat pluton consists of a medium- to commonly partly altered to epidote or chlorite. Optical coarse-grained hypidiomorphic-granular hornblende­ properties and accessory minerals are g~ven in table 2. biotite quartz monzonite. The rockisextremelyuniform Small rounded mafic inclusions composed mostly of in both mineralogy and· texture. Modal analyses show hornblende, biotite, and plagioclase are common, that the composition is also uniform, except along the though not numerous, throughout the pluton. Only southwest border where the rock is contaminated ap­ along the south border of the body where the quartz parently by assimilation of Huckleberry greenstone or monzonite intrudes greenstone of the Huckleberry For­ by reaction with it. The average modal composition of mation is the identity of these inclusions known. There the noncontaminated rock plots well within the quartz they are many times more numerous than in the inter­ monzonite field, but toward the granodiorite side. (See ior and are demonstrably xenoliths. In specimens col­ fig. 11.) lected within a few hundred yards of the contact, the In hand specimen the quartz monzonite is light gray average plagioclase composition is more calcic by about and commonly speckled with pink feldspar. White An10 than in the typical quartz monzonite, and mafic plagioclase, pink or white potassium feldspar, clear or minerals are almost 50 percent more abundant. Horn­ smoky quartz, shiny black biotite, greenish-black horn­ blende predominates over biotite in the border rock, blende, and golden-brown crystals of sphene are easily which is the reverse relationship of that in the bulk of seen without a hand lens. A distinguishing feature of the pluton. The average modal composition of "normal" this rock is the ubiquitous occurrence of biotite in and border-zone rock is as follows: euhedral pseudohexagonal tablets. Normal Border zone Plagioclase is the most abundant mineral in the rock. (nine specimens) (four specimens) Crystals are subhedral to euhedral and average about Plagioclase ------37 43 0.15 inch in length, although some reach 0.4 inch. Aver­ Potassium feldspar ------24 15 Quartz ------26 23 age composition is An2o• The most calcic plagioclase Mafic minerals ------13 19 core measured is An35' and the most sodic rim An2o· Mafic minerals in border specimens are concentrated Some crystals appear to be unzoned, whereas others are in small clusters,: along wi.th sphene, apatite, and highly complex and show both normal and reverse opaque minerals. In addition, clinopyroxene (Z 1\ C= zoning. 37°) is present in the cores of some hornblende crystals. Most of the potassium feldspar appears to be ortho­ During emplacement, chemical and physical condi­ clase. Perthitic intergrowths are common but extremely tions in the magma apparently allowed rapid assimila­ fine and difficult to see even under very high magnifica­ tion of the greenstone. This is reflected not only by the tion. Carlsbad twinning is present but rare. Crystals larger number of inclusions near the contact, but also are anhedral and occur interstitially to all other min­ by compositional and mineralogical differences between erals of comparable grain size except quartz. Average the contaminated marginal rock and the relatively size is about 0.2 inch; some grains are as large as 0.6 uncontaminated interior. The contamination effects inch. No compositional zoning was noted, but small appear to extend less than 1 mile into the interior of crystals of plagioclase, quartz, hornblende, biotite, and the pluton, and diminish most rapidly within the first apatite are oriented parallel to crystallographic direc­ 500 feet from the contact with the greenstone. tions in some crystals. Quartz also occurs as anhedral grains, filling inter­ OTHER ROCKS INCLUDED WITII TilE stices between plagioclase and the mafic minerals. Crys­ STARVATION FLAT QUARTZ MONZONITE tals average about 0.14 inch in size. Undulatory extinc­ A rock of unknown extent, which may be either a tion is present in all grains but is not extreme. The porphyritic phase of the Starvation Flat Quartz Mon­ quartz is clear in thin section and contains only a few zonite or a separate plutonic mass, crops out due north almost submicroscopic inclusions. of Starvation Lake and for several miles along State Mafic minerals average 13 percent of the Starvation Highway 294 north of the area. Although its ground­ Flat Quartz Monzonite. The hornblende to biotite ratio mass texture is similar to that of the Starvation Flat averages about 0. 7 except in the vicinity of the Huckle­ Quartz Monzonite, the rock contains phenocrysts of berry greenstone where hornblende is slightly more microcline as much as 3 inches long and almost no abundant. Biotite occurs as subhedral and euhedral hornblende. In all other respects, including optical crystals averaging about 0.12 inch across. mineralogy and accessory minerals, the rock is similar Hornblende is present in every specimen. Individual to the Starvation Flat Quartz Monzonite. If this rock crystals are from about 0.04 to 0.16 inch long. Crystals is not a phase of that pluton, the similarities suggest are subhedral to euhedral and have reasonably consis- that it may be at least genetically related. 40 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

A leucocratic dikelike body intrudes the Starvation PJDLLIPS LAKE GRANODIORITE Flat Quartz Monzonite in sees. 34 and 35, T. 35 N., R. (AND ASSOCIATED DIKES) 40 E., and the two sections immediately to the south. LOCATION, EXTENT, AND TOPOGRAPIDC EXPRESSION In outcrop the rock resembles aplite and contains The name Phillips Lake Granodiorite is here coined small pod-shaped bodies of coarser grained material. for the muscovite-biotite granodiorite exposed so well The coarser grained pods are less than 1 inch in diam­ around Phillips Lake, its type locality, in sec. 34, T. 34 eter, but some are as much as 4 inches long and crudely N., R. 41 E. This pluton underlies about 60 square miles tabular. In addition, the rock contains small miarolitic in the northeast corner of the report area. A plutonic cavities. Most of the rock is pale pink to light gray. rock that is mineralogically, texturally, and modally Biotite, the only dark mineral, makes up less than 1 similar has been found 17 miles north of the area on percent of the body. The rock is unusual in that the Huckleberry Mountain in the Spirit quadrangle (Yates fine~grained part is a mass of graphic intergrowths of and Engels, 1968, p. D245). To the east, the granodio­ quartz in albite and potassium feldspar (fig. 12). The rite is found in the Tacoma Creek drainage in the graphic texture, however, is not easily detected in hand northwest quarter of the Newport 30-minute quad­ rangle. The full extent and configuration of this body in the areas to the north and east have not yet been determined, but the pluton appears to underlie a much larger area outside the quadrangle. Unlike other plutons in the report area, the Phillips Lake Granodiorite underlies the highest elevations, and it appears to be considerably more resistant than any of the other plutons. However, this may be due largely to the more resistant remnants of metamor­ phosed Precambrian roof rocks and the leucocratic dikes which inject the pluton. The divide which runs partly outside the east boundary of the area from north of Calispell Peak south to Goddards Peak is capped by small but numerous roof remnants of metamorphic rock. Most of these appear to be less than 500 yards long and in this part of the area consist of amphibolite FIGURE 12.- Photomicrograph of the fine-grained dike-form mass of graphically intergrown rock that intrudes the Starva. and mica.schist. t~on Flat Quartz Monzonite. The texture is typical of the INTERNAL FEATURES entire rock, not just an isolated area. Long dimension of photograph is about 2.5 mm. Crossed nicols. The Phillips Lake Granodiorite is easily distinguish­ able from all other intrusive rocks in the report area. It specimens. None of the rock has the xenomorphic tex­ is dominantly a biotite-muscovite granodiorite but ture typical of aplitic dikes. The average modal compo­ varies greatly in composition (fig. 11). The interstitial sition of four specimens is as follows: arrangement of the muscovite and biotite flakes among Albite ...... 28 the larger crystals of anhedral quartz and subhedral Orthoclase (microperthite) ...... 36 plagioclase is distinctive in hand specimen as well as in Quartz ...... 36 thin section (fig. 13). The micas in much of the rock ·Biotite ...... Trace impart a slight foliation, which was probably formed The homogeneity of the rock is indicated by the indi­ before the pluton totally solidified. vidual modes, none of which vary more than 2 percent The granodiorite is intruded almost everywhere by from the average. Although an unknown, but probably leucocratic dikes which fall into three general groups. small, amount of error is introduced, the mode can be The most abundant are medium- to fine-grained equi­ used to approximate a norm because the plagioclase granular quartz monzonite dikes from less than 1 inch composition is only Anz, and the microperthitic potas­ to over 200 feet thick. Where most numerous, in the sium feldspar contains few lamellae of albite. The mode eastern part of the pluton, they average about 50 feet

plots in the SiOz-NaAlSi30 a-KAlSiaOs minimum trough thick and appear to be randomly oriented. The dike

for 2,000 bars H 2 0 pressure (Tuttle and Bowen, 1958, walls are roughly parallel and relatively planar. The p. 55), but falls about 10 percent off the ternary mini­ quartz monzonite also forms irregular bodies, the larg­ mum point for that pressure in the direction away from est of which is a small cupola about 1 mile in diameter

the NaAlSi30 8 corner. that crops out northwest of Lenhart Meadows (sec. 23, MESOZOIC PLUTONIC RCCKS 41

1 percent of the area shown as Phillips Lake Granodio­ rite. They consist of aplites and pegmatites of the more typical varieties. Pure white aplite dikes, without bio­ tite, occur throughout the pluton but are especially common around the margins. Although they have rela­ tively planar walls, these dikes are considerably more irregular in shape than the quartz monzonite dikes. In hand specimen, microcline, plagioclase, quartz, musco­ vite, garnet, and tourmaline are visible. Pegmatities are irregularly intermixed with the aplites and also, to about the same degree, with the quartz monzonite dikes. The association with the latter is largely restricted to the margin of the pluton. Most of the pegmatites consist only of perthite, sodic plagio­ clase, quartz, and muscovite_, but some also contain biotite, garnet, tourmaline, and rarely beryl, or colum­ bite. Where associated with the biotite-bearing quartz monzonite dikes, the pegmatites commonly contain intergrowths of muscovite and biotite in the same pseudohexagonal tablet. The three dike types are found together in only a few places, but where they are, the pegmatites and aplites both cut the quartz monzo­ nite dikes. From Phillips Lake eastward to the border of the report area, all three dike types become increasingly more numerous, and in the vicinity of Calispell Peak, the granodioritic country rock makes up probably less than half the rock exposed. This increase in the ratio of dikes to host rock is accompanied by a decrease in the potassium feldspar content of the granodiorite from an average of 15-20 percent near Phillips Lake to 0-5 percent along the east border. Also, the foliation in the FIGURE 13- Stained slab of Phillips Lake Granodiorite. Feld­ granodiorite, barely perceptible at Phillips Lake, be­ spar is white, quartz dark gray, and biotite black. Even comes increasingly more pronounced toward the east though the specimen contains only about 5 percent potassium border. feldspar, it is not foliate as is typical of most potassium feld­ spar-deficient parts of this body. The specimen shows the The concentration of dikes thus may be related to interstitial occurrence of the biotite around the felsic miner­ the degree of foliation and to changes in composition of als. Muscovite, although it does not show in the photograph, the host rocks. Several hypotheses were considered to has the same relation to the quartz and feldspar as the biotite. explain these relations. The most probable explanation Specimen is 3.5 inches long. is that during the later stages of crystallization, when the composition of the remaining melt was similar to T. 34 N., R. 41 E). Concentrations of quartz monzo­ that of the dikes, the alkali- and volatile-rich melt was nite float suggest similar bodies may be present in the removed from the interstices of the already crystallized northeastern part of the Phillips Lake Granodiorite. material, perhaps by filter pressing. The mobilized melt The general distribution of the quartz monzonite dikes formed the dikes; thus, the granodiorite is most defi­ is shown on the geologic map by an overprint. Where it cient in potassium feldspar where the dikes are most intrudes sedimentary or metamorphic rocks, this rock numerous. The foliate texture results from the collapse invariably forms sills. Poor exposures prevent precise accompanying removal of the melt, which forced the calculations of the total area underlain by the equi­ micas and remaining melt into interstices between the granular quartz monzonite, but the area should be at larger quartz and plagioclase crystals. least a third of that mapped as Phillips Lake Granodio­ The partitioning of potassium between microcline, rite. muscovite, biotite, and plagioclase is also interpreted Dikes belonging to the other two groups are rela­ to mean that part of the melt was physically removed tively few in number and underlie probably less than from the granodiorite. Neither the muscovite nor biotite, 42 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON the only other two mineral phases in the granodiorite are cut by nonmineralized joints, suggesting more than containing appreciable amounts of potassium, system­ one generation. Joint attitudes were not systematically atically changes in amount with the decrease in potas­ measured and recorded on the map because most of the sium feldspar content in the eastern part of the pluton. rounded outcrops into which the granodiorite char­ Plagioclase compos.ition does not change systematically acteristically weathers have obviously been rotated or either. moved by frost heaving. The bulk composition of the original magma would CONTACT RELATIONS have been considerably more alkali rich than the com­ position of the Phillips Lake Granodiorite indicates if The contact between the Phillips Lake Granodiorite all the dikes were originally derived from· the Phillips and the metamorphosed host rocks is exposed only in a Lake magma as suggested. The presence of two micas few roadcuts, and in some of these, injection of the leu­ as characterizing minerals in the granodiorite supports cocratic dikes has been so intense as to obliterate con­ this suggestion. Muscovite and biotite are not the char­ tact relations. A few generalizations can be made, acteristic minerals one would expect in a rock of this however. composition, especially along the east border of the Possibly the most striking feature of the contact is its area where the feldspar ratio of the host rock is that of generally shallow dip. The outcrop pattern on the map a quartz diorite. However, a theoretical composition shows a very low angle dip on the north flank of Wilson resulting from the combination of the magmas from Mountain and around Bell Meadow. The southeastern­ which the granodiorite and the leucocratic dikes crys­ most 2 miles of the contact was drawn with very little tallized would be one from which muscovite and biotite control because of poor exposure and heavy forest might crystallize. cover. However, the presence of highly recrystallized To arrive at a weighted-average mode for the grano­ schist with porphyroblasts of andalusite as much as 4 diorite and dikes combined, the ratio of dike rock to inches long in the saddle north of Goddards Peak sug­ pluton was estimated from surface exposure. As close gests that the pluton underlies this area at shallow as can be estimated, about 35 percent of the area shown depth and therefore that this segment of the contact is as granodiorite on the map is underlain by dike rock. indeed shall9w. Almost all the roof remnants in the By using this estimate, the average modal compositions eastern part of the pluton also have shallow-dipping of the granodiorite and the dike rocks were weighted contacts. From the vicinity of Bear Canyon south to proportionately, and the following mode calculated: The Tinderbox, the contact appears to be steeper. Where exposed, the contact is highly irregular owing Plagioclase ...... 40 to numerous dikes of both granodiorite and the leuco­ Potassium feldspar ...... 17 cratic rock. Along most of the contact the host rocks Quartz ...... 30 Mafics (including muscovite) ...... 13 appear to have reacted relatively passively; one excep­ tion is the large internal septum underlying McDonald This mode still plots in the granodiorite field, but near and Brewer Mountains, which was probably rotated in the granodiorite-quartz monzonite boundary. If the· a counterclockwise direction by the magma. volume of dike rock is actually greater than that of the The Precambrian host rocks surrounding the Phillips granodiorite, an original magma from which they could Lake Granodiorite show contact metamorphic effects have crystallized would have a quartz monzonite com­ for several miles from the surface trace of the contact. position. Because of the shallow dip of the contact, none of the Except for the large remnants of roof rock in the Precambrian rocks south of the pluton are very far eastern part of the pluton, almost no inclusions of any from the source of metamorphism. In the area between kind have been found within the Phillips Lake Grano­ the Phillips Lake Granodiorite and the Flowery Trail diorite or the associated dike rocks. Even near the Granodiorite, calc-silicate minerals have formed in all bo.rders, the only perceptible difference from the rock the carbonate-bearing rocks except those on the west in the interior, with respect to foreign material, is an flank of Eagle Mountain and at higher elevations on the ·increase in the amount of micas. mountain. Contact metamorphic effects along the seg­ Brecciation and shearing resulting from faulting have ment of the contact west of The Tinderbox but south of been recognized only along the northwest border of the the northwest-bordering fault do not appear to extend Phillips Lake Granodiorite and west and south of Bear as far into the host rocks as they do south of the pluton. Canyon, where the rock is highly fractured in places In the McHale Slate, which is intruded by the grano­ and recemented with quartz, chlorite, and epidote. diorite at The Tinderbox and on the hill immediately More than one set of joints is found in many outcrops to the north, recrystallization is not apparent at dis­ of the Phillips Lake Granodiorite. Mineralized joints tances greater than half a mile from the contact. This is MESOZOIC PLUTONIC ROCKS 43 probably because the contact is steep here but may be the granodiorite. Crystals average 0.12-0.2 inch in related to the greater distance from the Flowery Trail length and are mostly subhedral. Composition averages Granodiorite. The latter, which is an older intrusive, about An22· Normal zoning with minor reversals is appears to have "prepared" the host rocks for the Phil­ present, although it is not obvious in all crystals. The lips Lake· Granodiorite. Whether this "preparation" most sodic zone measured is An1a and the most calcic consisted of preheating or was of a chemical nature is An37· The plagioclase is generally glass clear and free not known, as potassium-argon dates suggest that the of the normal haze of sericite and other late-stage alter­ Flowery Trail Granodiorite is about 100 m.y. (million ation products commonly found in plagioclase from years) older than the Phillips Lake Granodiorite. plutonic rocks, but it does contain small euhedral to Along the south boundary of the Phillips Lake subhedral crystals of clinozoisite and epidote similar to Granodiorite, recrystallization of the host rocks at the those in the Flowery Trail Granodiorite. These inclu­ contact has developed mineral assemblages indicative sions and the highly discordant ages obtained from of the transition between the albite-epidote-hornfels potassium-argon analyses of muscovite and biotite sug­ facies and hornblende-hornfels facies of Turner and gest that the granodiorite has been metamorphosed. Verhoogen (1960, p. 511). Mineral assemblages in a Potassium· feldspar content averages about 11 per­ single thin section of a metamorphosed carbonate-bear­ cent but is. highly variable, as the modes show (fig. 11). ing rock typically include albite, epidote, clinozoisite, Only microcline has been identified, but orthoclase may quartz, biotite, microcline, diopside (rare), and abun­ also be present. The crystals vary in size but are about dant medium-green amphibole that is highly birefring­ the same as the plagioclase. Locally, the rock is notably ent. Highly quartzose pelitic rocks form coarse-grained porphyritic, with microcline phenocrysts as much as 1 quartz-mica schists that only rarely contain aluminum inch long. Smaller microcline crystals are mostly an­ silicates. A typical assemblage includes quartz, albite, hedral and appear to be interstitial to other minerals. muscovite, biotite, tourmaline, and, in some specimens, As with the plagioclase, no alteration is apparent other andalusite. The assemblages developed in pelitic rocks than a slight clouding in some crystals. Small crystals are clearly indicative of the albite-epidote-hornfels of muscovite are common within potassium feldspar facies, but the appearance of diopside and a hornblende­ crystals in some thin sections and probably formed dur­ like amphibole in the ca:.;bonate-bearing rocks suggests ing metamorphism. the transition to the hornblende-hornfels facies. · The quartz is a very distinctive pale gray violet and In the roof remnants. along the east border of the most commonly occurs in clusters of several grains area, the rocks are well into the hornblende-hornfels which look like one large crystal. Clusters are from 0.2 facies. As most of the rocks were originally carbonate to 0.4 inch across and are roughly oval in cross section. bearing, a typical assemblage includes diopside, horn­ Inclusions are rare. Myrmekitic intergrowth of quartz blende, quartz, and plagioclase of intermediate compo­ and plagioclase are common. sition. On the spur trending northwest from Calispell Biotite and muscovite crystals average about 0.04 Peak, a small roof remnant contains the assemblage inch in length and occur in clusters as much as 0.4 inch vesuvianite, scapolite, diopside, quartz, calcite, clino­ across. Commonly both micas rim or mantle either zoisite, and plagioclase of intermediate composition. quartz or feldspar. All crystals are subhedral and show At distances greater than 2,000 feet from any con­ almost no signs of alteration. Biotite is abundant and tact of the Phillips Lake Granodiorite, all rocks are widespread, but muscovite is locally almost absent. The well down into the albite-epidote-hornfels facies, but total amount of mica averages about 15 percent of the almost all are, nevertheless, thoroughly recrystallized. rock; the muscovite to biotite ratio averages about 0.3. The difficulties in separating the contact metamorphic Epidote is found in all speCimens but consistently effects of the Phillips Lake Granodiorite from those of amounts to less than 1 percent of the rock. Crystals the Flowery Trail Granodiorite to the south have al­ are small, averaging about 0.008 inch. Some of the rea.dy been discussed. See section "Flowery Trail epidote may be primary, as many of the larger crystals Granodiorite, Contact Relations." Pronounced meta­ have conspicuous cores of allanite. morphic effects do not extend more than about half a Apatite, garnet, magnetite, zircon, and tourmaline mile south of the Flowery Trail Granodiorite, and so occur as accessory minerals, but only apatite and zir­ most of the metamorphic effects more than 1 mile north con are widespread. Apatite is most abundant, and some of that pluton are probably due to the Phillips Lake crystals are as much as 1 mm long. Magnetite is rare Granodiorite. or absent in almost all specimens. Gamet and tourma­ PETROLOGY line are present in only a few specimens and are probably the product of metasomatism accompanying Plagioclase is by far the most abundant mineral in instrusion of nearby pegmatites or aplites. 44 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

TWO-MICA QUARTZ MONZONITE quartz monzonite which is almost identical in modal composition, texture, and appearance with the one in LOCATION, EXTENT, AND TOPOGRAPIDC EXPRESSION the report area. If all three two-mica bodies are geneti­ The west edge of a two-mica quartz monzonite pluton cally related, they represent a widespread plutonic crops out on the east flank of Nelson Peak. An area of entity in northeastern Washington. about 2 square miles along the east border of the report CONTACT RELATIONS area is underlain by the pluton. The total extent is not known, but anomalously gentle slopes continue for The contact between the two-mica quartz monzonite about 4 miles to the southeast in the adjacent Newport and the Prichard Formation is nowhere exposed, but quadrangle and indicate a surface exposure consider­ the relation of topography and inferred contact config­ ably greater than that within the report area. This uration suggests that it is steeply dipping. quartz monzonite, like most of the plutonic rocks in Contact metamorphic effects are noticeable 1 mile the area, forms a topographic low and gentle slopes southwest of the·pluton but cannot confidently be dis­ except where in contact with more resistant metamor­ tinguished from those caused by the coarse-grained phic rocks. quartz monzonite immediately to the south. Just east of Nelson Peak, argillite 200 feet from the quartz INTERNAL FEATURES monzonite has been recrystallized to a quartz-musco­ Muscovite and biotite characterize this pluton. They vite-biotite hornfels with small porphyroblasts of chlor­ were found everywhere that the rock was examined. ite and garnet. The chlorite shows signs of reaction and The ratio of muscovite to biotite averages about 0.30 probably became unstable when the garnet began crys­ and varies only slightly from place to place. The Phil~ tallizing. Although a contact metamorphic occurrence, lips Lake Granodiorite and its associated dikes are the the assemblage is most like the quartz-albite-epidote­ only other rocks in the area that contain two micas. almandine subfacies of the greenschist facies ( Fyfe and The quartz monzonite has a hypidiomorphic-granular others, 1958, p. 224). texture and is uniformly coarse grained. It is almost PETROLOGY invariably deeply weathered and friable. The· feldspars have a bleached appearance, and biotite is commonly The plagioclase in this rock is oligoclase. The crystals surrounded by a brown stain of iron oxides. are subhedral to euhedral and commonly have sodic Texturally and mineralogically the rock appears to rims and calcic cores. Some plagioclase crystals contain be quite uniform, but exposures are so poor that in­ crystals of biotite and muscovite. Large parts of .some ternal variations could easily go unnoticed. No dikes or plagioclase crystals are replaced by irre~lar-shaped inclusions were found, and exposures are not such to grains of microcline. show joint development. All potassium feldspar appears to be microcline. The Modal analyses of seven specimens from different characteristic grid ·twinning is well developed. The localities scatter on both sides of. the quartz monzonite­ potassium feldspar does not occupy interstices between granodiorite boundary (fig. 11); however, the average other minerals, as it. would if it were a late-stage filling, composition, shown below, is on the quartz monzonite but occurs as subhedral crystals with the same relation side and is close to the calculated average modal compo­ to surrounding minerals as the plagioclase. In this sition of the Phillips Lake Granodiorite and associated respect it is different from that in most of the plutonic leucocratic dikes. (See section "Phillips Lake Grano­ rocks of the report area. diorite (and associated dikes), Internal Features.") Large gray anhedral knots of quartz fill spaces be­ tween other minerals. The quartz contains inclusions B of all other minerals in the rock and shows little undula­ Plagioclase ...... 38 40 tory extinction. Potassium feldspar ...... 21 17 Quartz ...... 30 30 Muscovite and biotite are randomly oriented and not Ma:fics (includingmuscovite) ...... 11 13 wrapped around felsic minerals as in the Phillips Lake 1. Average modal composition of two-mica quartz mon­ Granodiorite. They most commonly occur in patches in zonite. 2. Calculated average modal composition of Phillips which the two minerals are intergrown with one Lake Granodiorite and associated leucoeratic dikes. another. Biotite has the pleochroic formula X=golden If not just coincidental, the similarity suggests that the tan, Y =Z==reddish- brown. two plutons are related or are disconnected parts of the Apatite and zircon are the only common accessory same pluton. minerals in the rock. Zircon is unusually abundant and In the Rattlesnake Hills, 15-20 miles south-southeast dots the biotite with pleochroic halos. Opaque minerals of Loon Lake, Griggs ( 1966) mapped another two-mica are unusually scarce. MESOZOIC PLUTONIC ROCKS 45

COARSE-GRAINED QUARTZ MONZONITE it represents modal analyses of several large samples from each locality. LOCATION, EXTENT, AND TOPOGRAPIDC EXPRESSION Samples from a number of places in sees. 18 and 19, An area of about 15 square miles in the southern part T. 30 N., R. 42 E., show a pronounced bimodal grain of the report area is underlain by coarse-grained biotite size. Crystals of potassium feldspar, quartz, plagioclase, quartz monzonite. The pluton has a highly irregular and biotite averaging about 0.2 inch in size make up outline and crops out in two separate areas divided by about 65 percent of the rock and are set in a ground­ a septum of Belt rocks just east of Deer Lake. The east­ mass of the same minerals with an average grain size of ernmost of the two areas extends beyond the report about 0.05 inch. This pronounced textural variation area for an unknown distance. may be due to thermal or pressure quenching of a crys­ The pluton is a perfect example of how easily the tal-magma mixture. These textural variations could plutonic rocks in this region are eroded relative to the not be mapped, but th~y have been found only on and metamorphic rocks. The part of the pluton northeast around the projections of preintrusion faults (pl. 2). of Deer Lake forms an open-ended basin, and a notice­ As thin sections show no cataclasis, the texture is pre­ able break in slope marks almost the entire length of sumed to be a primary feature and is probably due to the contact in this area. South of Deer Lake and north­ some deviation from the normal crystallization condi­ east of Loon Lake, almost the entire axis .of this part tions of the pluton. The melt may have intruded host of the pluton is marked by prominent valleys. North rock that was shattered enough so that it could not and west of Loon Lake the pluton forms low hills or retain the fluid pressure maintained by the rest of the underlies fiat alluviated areas. intrusive. Upon loss of fluid pressure, relatively rapid Most of the rock is deeply weathered. In numerous crystallization took place locally and possibly sealed 10-15-foot roadcuts, in all parts of the area, only loose the system from any further leakage. friable rock can be found. Natural exposures are rare, Leucocratic muscovite-bearing bimodal rocks are and most of them are highly weathered. Only on State present at the outer margins of the pluton, near the highway 292, 3% miles east of Springdale, and on the contact. These, however, are finer grained than the road between Loon Lake and Deer Lake is the rock bulk of the pluton. reasonably fresh. Inclusions of any sort are not abundant in this plu­ INTERNAL FEATURES ton, but dikes\are common, especially around the mar­ gins. Some are normal-looking aplites and fine-grained The coarse-grained quartz monzonite is easily identi­ muscovite-bearing rocks with graphic texture. These fied by grain size: The average size is greater than half are probably genetically related to the quartz monzo­ an inch. Grains of quartz and feldspar are about equal nite because of their consistent spatial relationship. in size, but grains of biotite are only about %-% inch Others, which consist of a dark-green rock, are highly across. altered. They also cut other plutons and may not be Although most of the rock has a hypidiomorphic­ related to this pluton. granular texture and is not obviously porphyritic, phenocrysts of pink potassium feldspar as much as 2 CONTACT RELATIONS inches long are locally common. Even where the rock is The attitude of the contact could not be measured nonporphyritic, the abundant potassium feldspar im­ directly, but the relationship of its configuration to parts a pink cast. topography suggests that it dips outward between 40° Locally, the distribution of potassium feldspar is not and 50° in most places. uniform. In places, as much as a cubic yard of rock may Contact metamorphic effects of the quartz monzonite consist of 50-60 percent potassium feldspar. Pods of are pronounced within a few hundred feet of the con­ potassium feldspar are irregular in shape and common tact but fade out rapidly beyond that. Good exposures throughout the pluton. Textural relations and grain of metamorphosed siltite and argillite on J umpoff Joe size within these pods are the same as those in the rest Mountain, Deer Lake Mountain, and on the ridge east of the quartz monzonite. The pods may represent crys­ of Benson Peak furnish excellent control for determin­ tal accumulates. Alternatively, they may have formed ing the degree of metamorphism caused by the quartz by some sort of localized filter pressing process or may monzonite. At all three localities the range in chemical be metasomatic. composition of the host rocks is about the same. Other The nonuniform mineral distribution makes it diffi­ physical and chemical conditions must have been uni­ cut to estimate the overall composition of the pluton form also, because the aureoles are nearly the same with accuracy. The average composition shown in figure width at all three localities and the minerals that crys­ 11, however, is probably reasonably accurate because tallized are almost identical. Less than 200-300 feet 46 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON from the contact, the most common mineral assemblage lies as much as 3 square miles outside the area. A. B. is andalusite-cordierite-biotite-muscovite-quartz> in­ Griggs located small outcrops of muscovite quartz mon­ dicative of the hornblende-hornfels facies of Turner zonite at two localities just south of the report area. and Verhoogen ( 1960, p. 513). Beyond 200-300 feet, One is 4 miles south of Clayton, the other 3 miles south­ andalusite and cordierite ~re absent. east of Deer Park (oral commun., 1968). The rocks at Quartz-albite-muscovite-biotite of the albite-epidote­ both localities are identical in every respect and are hornfels facies is the typical assemblage in the interval undoubtedly genetically related to the muscovite from 200 or 300 to about 600 feet. All the rocks in this quartz monzonite in the report area. In addition, Griggs interval are thoroughly recrystallized but do not con­ reported that the same rock type crops out at various tain any of the higher grade minerals. Beyond 600 feet, places in the mountainous part of the Clayton quad­ chlorite porphyroblasts, fine-grained muscovite, incipi­ rangle. Although individual plutons appear to be rela­ ent biotite, albite, and quartz are the characteristic tively small, the rock type is apparently widespread in minerals. Much of the rock beyond 600 feet does not the region. appear to be thoroughly recrystallized. Beyond about Like most of the other plutonic rocks in the area, the 1,500 feet, few obvious metamorphic effects. are observ­ muscovite quartz monzonite forms topographic lows. able. The contact of the pluton with the more-resistant PETROLOGY metamorphosed host rock generally is a break in slope. Although exposures are generally poor, contacts can be The mineralogy of the coarse-grained quartz monzo­ nite is relatively simple and uniform. Plagioclase, the reliably mapped on the basis of this break in slope and most abundant mineral, averages An2o and occurs as the presence in the soil of mica from the decomposed plutonic rock. white euhedral to subhedral crystals ranging in size from about 0.2 to 0.8 inch. Most crystals show some CONTACT RELATIONS zoning, but it is not particularly pronounced. Locally Other than small variations in grain size, the musco­ the plagioclase is sericitized and is pale green in hand vite quartz monzonite at the contact is identical with specimen. that in the interior. As with some of the other plutons, Microperthitic orthoclase crystals are subhedral to the metamorphic effects on the country rock are diffi­ anhedral and range in size from about 0J2 inch to more cult to separate from those of other plutons nearby. On than 2 inches. One of the characteristics of the rock, the Blue Grouse Mountain, which is almost 1 mile from any significance of which is not understood, is the presence any other pluton, the contact metamorphic effects are of a thin hairline film of potassium feldspar around very slight. At the contact, siltite has been recrystal­ most plagioclase crystals. No evidence was seen to indi­ lized into medium-grained quartz-muscovite schist. cate whether this film resulted from exsolution or However, hand specimens of host rock from more than from crystallization of the late-stage potassium-rich 50 feet away show almost no metamorphic effects, and solutions. thin sections of this rock show only slight recrystalliza­ Biotite constitutes about 7 percent of the rock and tion. is the only mafic mineral. It is generally finer grained The south border of the largest muscovite quartz than the other major minerals. The borders of most monzonite intrusion in the report area is cut by numer­ crystals are slightly chloritized, but the centers are ous huebnerite-bearing quartz veins, as is the adjacent unaltered. host rock. Parts of the contact are greisenized, and the Some sphene crystals are as large as a quarter of an quartzite and siltite as much as several hundred feet inch across and easily seen in hand specimen. The other from the contact are pock marked where limonite accessory minerals, given in table 2, are normally fine . psuedomorphs have been leached out. These veins and grained and less abundant. the associated mineralization appear to be related to the quartz monzonite. MUSCOVITE QUARTZ MONZONITE PETROLOGY LOCATION, EXTENT, AND TOPOGRAPmC EXPRESSION On the basis of mineralogy, the rock is quartz monzo­ Leucrocratic muscovite quartz monzonite is exposed nite in composition and is referred to as such in this in three areas in the southeastern part of the report report. However, because the plagioclase is sodic albite, area and underlies a total area of about 2 square miles. the rock could be classified as granite chemically. The The largest exposure, in sec. 9, T. 30 N., R. 42 E., ex­ average mode, a chemical analysis, and CIPW norm of tends about 2.5 miles east of the report area and under- an apparently representative sample are as follows: CENOZOIC PLUTONIC ROCKS 47

Chemical analysis1 CIPWnorm Average of 6 modes zircon. The garnet is pale brownish red and ranges from (weight percent) (tveight percent) (volume percent) a trace to 2 percent. SiO. ------75.1 Q ...... 32.3 Plagioclase ...... 34 AhOa ------14.0 c -·--·--··- 1.4 Potassium Fe:.Oa ------.22 or ...... 27.2 feldspar ...... 33 CENOZOIC PLUTONIC ROCKS FeO ...... 16 ab ...... 34.7 Quartz ...... 27 MgO ...... 15 an ...... 2.4 Muscovite...... 6 SILVER POINT QUARTZ MONZONITE CaO ...... 53 en ...... 4 Na20 ...... 4.1 fs ...... 2 100 LOCATION, EXTENT, AND TOPOGRAPmC EXPRESSION KaO ...... 4.6 mt ...... 3 The Silver Point Quartz Monzonite underlies about HaO- ...... 06 il --···----- .1 30 square miles in the southernmost part of the report HaO + ...... 94 cc .1 TiOa ...... 04 area. It was named by Miller (1969) for exposures at PaOs ...... 99.1 Silver Point on the west shore of Loon Lake. Although MnO ...... 06 its total extent is not known, this pluton apparently COa ...... 05 underlies an extremely large area. The rock is exposed 100.01 in roadcuts along U.S. Highway 2 that are 15 miles 1Analyzcd sample collected in NE%, sec. 26, T. 30 N., R. 41 E. Analyzed east-northeast of Loon Lake and may extend beyond by P. L. D. Elmore, S. D. Botts, Lowell Artis, James Kelsey, Gillison Chloe, James Glenn, and Hezekiah Smith. there. South of the area, the pluton extends at least 2 The mineralogy of the rock varies little from place miles into the northern part of the Clayton quadrangle. to place. Although the proportion of the minerals varies Rock similar in appearance to the Silver Point Quartz sJightly, all specimens are made up of the same min­ Monzonite is found about 20 miles southwest of Loon erals. The only noticeable textural variations are slight Lake on the west side of the Wellpinit quadrangle but differences in grain size. Near the margins and in the may not belong to the same plutonic mass (A. B. smaller intrusions, the grain size is only about 0.15 Griggs, oral commun., 1968). inch, compared with about 0.2 inch for the bulk of the The Silver Point Quartz Monzonite forms areas of rock. The texture of the entire quartz monzonite is moderate to low relief. Almost all exposures are deeply hypidiomorphic-granular. Color is pink to cream, de­ weathered. Along U.S. Highway 395 near the south pending on how altered the potassium feldspar is. Small border of the area, some new roadcuts 20 feet deep do spots of limonite a few inches to a few feet apart are not penetrate unweathered rock. In the hills bordering present in most of the quartz monzonite. No dikes or Ahren Meadows, however, most of the quartz monzo­ inclusions were found anywhere in the rock. nite is relatively fresh, probably because of glacial ero­ Average modal analyses of this rock indicate that sion. The least weathered and best exposures are along potassium and sodium feldspar contents are almost the west and south shores of Loon Lake, where roadcuts equal. Miller (1969, p. 5) earlier reported the plagio­ have recently been blasted into very fresh rock. clase composition to be about An10, but additional work using immersion oils indicates an average composition INTERNAL FEATURES AND PETROLOGY of An8 • Normative An of plagioclase calculated from The Silver Point Quartz Monzonite is a porphyritic two analyses averages 3.5. The crystals are subhedral to hornblende-biotite quartz monzonite. One of the most euhedral, well twinned, and almost devoid of zoning. striking features of the rock is its lack of internal varia­ The rock does not appear to have been albitized. tion, either in composition or texture. Modal analyses Potassium feldspar is subhedral to anhedral pink or (fig. 11) plot in a very small field that would probably cream-colored microcline. In thin section most crystals be even smaller if slightly larger slabs had been counted. show the characteristic grid twinning. Most of the Several specimens collected 10-15 miles east of Loon microcline is microperthitic, but the sodic phase appears Lake do not vary more than 2 percent in any mineral to have formed from normal exsolution rather than by phase from specimens obtained around Loon Lake. albitization. Along with quartz, the microcline occupies Near the borders, the pluton shows no obvious con­ interstices between other minerals. tamination effects and no variability in modal compo­ Muscovite, the sole characterizing mineral, averages sition. Color index ranges from 14 to 21, but for almost about 6 percent of the rock. No textural features in all specimens it falls in a relatively narrow range be­ hand specimen or thin section suggest that any of the tween 14 and 18. The ratio of hornblende to biotite is muscovite is secondary. consistently between 0.75 and 1.0. The rock contains no mafic minerals other than The texture of the Silver Point Quartz Monzonite is specks of magnetite, pyrite, or limonite. Accessory min­ unlike that of any other plutonic rock in the report area. erals, in order of abundance, are garnet, apatite, and The distinctive feature is the groundmass, which is ·> 48 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON composed of two size groups of crystals (fig. 14): Crys­ The table following compares the average modal com­ tals of hornblende, biotite, plagioclase, potassium position of four specimens on the basis of the size range feldspar, and quartz averaging 0.1-0.2 inch in size make of crystals: up about 40 percent of the rock; the rest of the ground­ Four Four specimens, mass consists of crystals averaging only 0.02-0.06 inch Four specimens, groundmass Entire specimens, ground.mass recalculated in size and is composed of all mineral phases found in pluton entire rock only to 100 percent the rock. This bimodal texture, like the modal compo­ (a) (b) (c) (d) d-b sition of the quartz monzonite, varies little from place Plagioclase ... .40 42 13 21 -21 to place. The greatest element of textural variability is Potassium feldspar .... 25 26 22 36 the size and content of potassium feldspar phenocrysts, +10 Quartz ...... 18 18 15 25 + 7 which range from * to 172 inches and from less than 1 Mafics ...... 17 14 11 18 + 4 to about 5 percent. Large crystals ...... 39 The specimens are from the southeast end of Loon Lake. As the last column shows, the proportion of plagioclase in the groundmass is considerably lower than it is in the rock as a whole, whereas the proportion of potassium feldspar, quartz, and mafic minerals is higher. If the larger crystals are considered to be earlier crystallization products, as they probably are, then the different proportion of minerals in the two size groups would suggest that a normal but pronounced differen­ tiation occurred during crystallization. The average modal composition of the groundmass is plotted in figure 11 for comparison with the average composition of the rock as a whole. Some event, such as a rapid loss of heat or volatiles, caused approximately the last 60 percent of magma to crystallize so rapidly that it could not react with already crystallized minerals. The Silver Point Quartz Monzonite shows none of the features interpreted as recrystallization effects in the Flowery Trail Granodiorite and Phillips Lake Gran­ odiorite. Mafic minerals appear to have the same optical properties throughout the pluton, and the feldspars contain the haze of alteration products normally found in feldspar of plutonic igneous rocks. Several north- to north-northwest-trending shear zones have been recognized west of Loon Lake. The largest zone, immediately west of the lake, is 500 feet wide at one place and was traced for more than 4 miles. In sec. 31, T. 30 N., R. 41 E., one of the zones intersects the contact between the Silver Point Quartz Monzonite FIGURE 14.-Stained slab of Silver Point Quartz Monzonite (upper) and granodiorite from the small pluton southeast of and the fine-grained quartz monzonite, apparently Springdale (lower). The Silver Point specimen is a perfect without offsetting it. The exposures are so poor in this example of the unusual texture found throughout that pluton. area, however, that the contact could be offset as much The large felsic and mafic minerals are "floating" in a finer as half a mile in an apparent right-lateral sense. grained interstitial mass of the same mineral types. Sparse Within these zones, the rock is highly sheared and potassium feldspar phenocrysts are typical of the pluton. The granodiorite specimen shows the high color index and equi­ displays cataclasis. Thin, anastomosing seams of silica granular texture typical of this body and contrasts sharply and chlorite bond the rock and make it extremely with the texture of the Silver Point specimen to which it is strong. Small aplite dikes within the zones are highly probably genetically related. Plagioclase is white, potassium broken and are offset in a right-lateral sense along indi­ feldspar gray, and hornblende and biotite black. Quartz is too fine grained to show well. The Silver Point specimen is vidual shears. In thin section all minerals appear highly 5 inches long, and the slabbed face of the granodiorite 3 granulated and almost all mafic minerals appear to inches long. be ground up and chloritized (fig. 15). The highly CENOZOIC PLUTONIC ROCKS 49

mens it is pink and easily distinguished from the plagioclase. Quartz makes up about 18 percent of the rock but is difficult to see without a hand lens, as most grains are less than 0.04 inch long. It shows undulatory extinc­ tion and is interstitial to all other minerals. Hornblende occurs as subhedral to euhedral crystals as much as 0.4 inch long. Average size is about 0.12 inch. Most shows no alteration. The optical properties shown in table 2 were checked in six samples from vari­ ous localities, and little variation was noted. Biotite, which is present throughout the pluton, is closely associated with hornblende. More biotite is pres­ ent in the fine-grained groundmass than in the coarse. Only a few biotite crystals are as much as 0.12 inch in size, whereas most of the hornblende is this size or larger. Sphene is the most obvious accessory mineral and is easily seen in hand specimen. Sphene, magnetite, and less conspicuous apatite occur in about equal propor­ tions. Small zircon crystals are scattered throughout the rock but are not abundant. Allanite, relatively rare, occurs in crystals as much as 0.08 inch long.

CONTACT RELATIONS Almost nowhere is the contact between the Silver Point Quartz Monzonite and any other rock exposed. Even where contacts are shown on the map, the actual trace is almost invariably covered. Because of this, no field evidence was found to ascertain the relative ages B between the Silver Point Quartz Monzonite and the two smaller plutons adjacent to it west of Loon Lake. FIGURE 15.-Photomicrographs of Silver Point Quartz Mon­ Along this part of the contact, the quartz monzonite zonite. A, Typical undeformed Silver Point Quartz Monzon­ ite. Crossed nicols. B, Cataclastic Silver Point Quartz Mon­ exhibits no reduction in grain size, no foliation, and no zonite from the shear zone south and west of Loon Lake. Note increase in the number of inclusions normally found in that all the mafic minerals, so abundant in A, are not present; the rock. Neither of the two adjacent plutons exhibit presumably they have been broken and altered to chlorite and these features either, and no apophyses or dikes were opaque minerals. Plane-polarized light. Long dimension of found that might indicate relative age. both photographs is about 4 rom. On the south flank of Loon Lake Mountain, the Silver Point Quartz Monzonite intrudes the Revett, Burke, and St. Regis Formations, and just east of Loon Lake deformed rock in the shear zones grades into completely Mountain, it intrudes the lower part of the Wallace undeformed rock over a distance of 20 or 30 feet. Formation. The rocks are highly weathered here also, Plagioclase, the most abundant mineral, occurs as and none of the contacts are exposed. However, small subhedral to euhedral crystals in both the fine- and but resistant outcrops of metamorphic rocks are found coarse-grained fractions of the groundmass. Average within 100 feet of the most uphill occurrence of soil composition is about An20 , with zoning ranging from containing decomposed granitic debris. An1s to An3s· Most of the rock intruded ranges from silty argillite Potassium feldspar, which appears to be microper­ to quartzite. Within a zone approximately 400 yards thitic orthoclase, is the only mineral that forms pheno­ wide next to the pluton, the argillitic rocks have been crysts. It does not show the microcline grid twinning, converted to quartz-albite-muscovite-biotite schist. but its crystal symmetry was not checked by X-ray. Where the quartz monzonite intrudes the lower part of Phenocrysts are euhedral, but groundmass minerals are the Wallace Formation in sec. 32, T. 30 N., R. 42 E., anhedral and fill intergranular spaces. In fresh sped- the carbonate-bearing siltstone is recrystallized to a 50 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON fine-grained quartz-albite-diopside-garnet-scapolite Potassium feldspar appears to be microperthitic hornfels. The presence of albite in this group of miner­ orthoclase. No microcline twinning was observed, but als probably indicates a disequilibrium assemblage. the crystals were not checked for symmetry by X-ray Assuming an approximately constant pressure at the methods. The potassium feldspar, along with quartz, particular level in the host rocks now exposed, during occupies interstices between other minerals. the emplacement of the quartz monzonite the tempera­ Quartz is anhedral, clear of inclusions, and only ture could have increased through the stability range slightly strained. The granodiorite has less quartz than of the albite-epidote-hornfels facies and into that of the any pluton in the report area. hornblende-hornfels facies. However, because the albite The hornblende-to-biotite ratio is approximately 1:1. has not been converted to more calcic plagioclase, the Hornblende forms euhedral crystals, some of which temperature probably did not remain high long enough have partially altered pyroxene cores. Both mafic min­ for a stable assemblage indicative of the hornblende­ erals occupy the same textural relationships to other hornfels facies to form. At distances of more than half minerals, and neither is segregated from the other. a mile from the quartz monzonite, the host rocks show Sphene, by far the most abundant accessory mineral, almost no metamorphic effects. is obvious in all hand specimens. Apatite, zircon, and magnetite are abundant but visible only in thin section. GRANODIORITE CONTACT RELATIONS A small elongate body of hornblende-biotite grano­ The granodiorite is in contact with the Addy Quartz­ diorite 2 miles southeast of Springdale underlies an area ite and three other plutonic rocks. Contacts are poorly of about 1 square mile. The rock does not occur any exposed, like those of most of the other plutons. On other place in the report area and has not been reported the west and north sides of the body, the contacts with outside the quadrangle. Although this pluton and the the coarse-grained and the fine-grained quartz monzon­ Flowery Trail Granodiorite are similar modally and ites are not exposed but were mapped on the basis of mineralogically, their ages are considerably different, float. The southeast segment of the contact with the according to potassium-argon analyses. Silver Point Quartz Monzonite is locally exposed, but evidence bearing on the relative age of the two plutons INTERNAL FEATURES AND PETROLOGY is lacking. Because the configuration of the ·contact is The granodiorite is easily distinguished from other highly irregular, the dip is hard to estimate from the plutonic rocks in the report area by its high color index relation of the surface trace to topography. Along a and by the fact that it is equigranular. In addition, it small segment of the contact, the granodiorite intrudes exhibits hypidiomorphic-granular texture and is com­ the Addy Quartzite, but because the quartzite contains pletely unfoliated. The pluton is nearly uniform inter­ relatively few impurities, the pluton produced no obvi­ nally, with no obvious variations in mineralogy, grain ous recrystallization effects. In the SW cor. sec. 2, T. size, or texture. 29 N., R. 40 E., almost half a mile .from the contact, The equigranular texture and high color index impart the impure lowermost part of the Metaline Formation a salt-and-pepper appearance (fig. 14). Both feldspars 6ontains needles of tremolite. These are the only obvi­ are white, and quartz, although present, is not obvious. ous contact metamorphic effects attributable to the Abundant specks of honey-brown sphene averaging granodiorite. 0.05 inch in length occur in all specimens and make up as much as half a percent of some. Average grain FINE-GRAINED QUARTZ MONZONITE size of the majo~ minerals is 0.1-0.15 inch. Fine-grained hornblende-biotite quartz monzonite Small aplite dikes as much as 6 inches wide cut the underlies about 1.5 square miles between Springdale pluton but are not common. Rounded mafic inclusions and Loon Lake. The pluton is irregular in shape but from 0.5 to 6 inches in diameter are scattered through­ crudely elongate in an east-west direction. Rock of this out the pluton. Irregular-shaped, wispy ma.fic segrega­ type is not found at any other place in the report area tions as much as 1 foot across are found from place to and appears to be confined to this single pluton. place but are not abundant. In general, the rock is deeply weathered and very Plagioclase, the most abundant mineral, forms euhed­ poorly exposed. Roadcuts along State Highway 292 ral to subhedral crystals, whose average composition is about 3.5 miles east of Springdale show that the rock between An2s and Anao· Crystals are normally zoned is highly weathered more than 10 feet below the sur­ from about Anas in the cores to about An2o at the rims. face. Because of this deep weathering, part of the south In most specimens, both feldspars show very little border was mapped on the basis of sparse float and is alteration. not well located. DIFFERENTIATION OF THE PLUTONIC ROCKS 51 INTERNAL FEATURES AND PETROLOGY with other plutonic rocks. The southern contact is very The fine-grained quartz monzonite has a rather uni­ poorly exposed, and the relation of the age of this plu­ form hypidiomorphic-granular texture throughout but ton to that of the granodiorite and Silver Point Quartz varies somewhat in grain size. In railroad cuts along the Monzonite is not known, nor is the attitude of the tracks of the Burlington Northern Railroad about 3.5 contact. miles east of Springdale, dikes of this rock have chilled On the other hand, the contact with the coarse­ borders and cut the more extensively exposed coarse­ grained quartz monzonite is locally well exposed and grained quartz monzonite. In this area and along the appears to dip less than 50° N. The coarse-grained plu­ nearby roadcuts on Highway 292, the grain size is 0.02- ton is clearly older than the fine-grained one. No con­ 0.05 inch, but at the northwest end of Loon Lake, it tact metamorphic effects resulting from the intrusion of is about 0.1-0.15 inch. the fine-grained quartz monzonite have been observed. In hand specimen the rock is characterized by its relatively fine grain size, a salt-and-pepper appearance DIFFERENTIATION· OF THE due to the interspersed light and dark minerals, and a PLUTONIC ROCKS splotchy pink cast due to coloring of the potassium feldspar. In the preliminary report on the north half of the Small aplitic dikes and dark-green aphanitic dikes report area, the existence of an apparent differentiation are common but not abundant in the pluton. Large sequence in the plutonic rocks was suggested (Clark inclusions are rare, but the small inclusionlike clots are and Miller, 1968, p. 3). A report on the south half almost everywhere in the finer grained parts of the (Miller, 1969) suggested that two separate differentia­ pluton. tion sequences might exist and that it might be pos­ The mineralogy appears to vary little in the few thin sible to separate rocks belonging to the Kaniksu and sections examined. Average plagioclase composition is Colville-Loon Lake batholiths on the basis of which about An2o, but many of the crystals have pronounced apparent differentiation sequence a particular pluton zoning. In all specimens examined, at least some of the belonged to (Miller, 1969, p. 4 and fig. 2A). Potassium­ plagioclase crystals are noticeably larger than all other argon ages since determined for most of the plutonic crystals. Potassium feldspar is microperthitic ortho­ rocks show that the ages of plutons thought to belong clase and together with quartz occurs as small irregular to one apparent differentiation sequence span a period grains interstitial to the· other minerals. of 150 m.y. Unless a differentiating magma can exist Hornblende is subhedral and commonly contains for 150 m.y., it would appear that the previously pro­ cores of partially altered pyroxene. Its indices of refrac­ posed differentiation sequences are more apparent than tion are considerably lower than in other plutons, and real. its pleochroism is noticeably different: X == pale tan, Most of the muscovite-bearing rocks appear to form Y == pale olive green, Z == pale green. The hornblende­ a crude trend, and all appear to belong to a 100-m.y. to-biotite ratio is about 1: 1, as in the granodiorite and period of plutonic activity. The Starvation Flat Quartz the Silver Point Quartz Monzonite. Biotite is similar Monzonite also belongs to this age group even though to that found in the other hornblende-biotite-bearing it is a hornblende-biotite-bearing rock.. and it plots plutonic rocks. closer to the apparent differentiation sequence charac­ Hornblende and biotite occur in about equal propor­ terized by hornblende and biotite. (See fig. 11.) tions and make up about 18 percent of the rock. To a It seems more than just coincidental, however, that noticeable degree, they occur in clots. The clots are the average modal compositions of the plutons plot on larger and more apparent at some places than at others such regular curves, that all the hornblende and biotite­ and may be partially resorbed mafic inclusions derived bearing plutons plot on one curve, and that all the from the Huckleberry greenstone or the Precambrian muscovite and two-mica plutons plot on another. To sills of the Prichard Formation. This may explain the postulate a differentiating melt existing for over 150 highly discordant potassium-argon ages obtained from m.y. strains the credulity of even the. most imaginative the hornblende. geologist. However, a differentiation may have occurred Sphene, by far the most abundant accessory mineral, even though the parent material was not continuously can be seen in most hand specimens. Apatite, magne­ molten. It is possible that this parent material, regard­ tite, and zircon are common. Allanite is present but not less of whether it was derived from igneous, sedimen­ as abundant as the other accessory minerals. tary, or metamorphic rocks, melted more than once ~thout a change in the factors controlling differentia­ CONTACT RELATIONS tion. During the quiescent periods between melting, The fine-grained quartz mo.nzonite is in contact only . differentiation of the parent material, and emplacement 52 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

of the individual plutons, chemical factors could have effects extend several tens of miles from the surface been essentially constant. Then, when a new period of outcrop, or else part of the pluton underlies at shallow melting occurred, differentiation could have begun depth the area north and northwest of where it is pres­ where it had left off at the end of the preceding period. ently exposed. Hart (1961, 1964), Doe and Hart (1963), and Han­ POTASSIUM-ARGON AGES son and Gast (1967) studied the effects of younger OF THE PLUTONIC ROCKS intrusions on host rocks, using a number of dating methods. They found that the apparent ages of the By JOAN C. ENGELS host rocks range from their true age to the age of the Seven of the eight major plutons in the report area intrusion, and they plotted apparent age of the host have been dated by the potassium-argon method, in rock against distance from the contact. In any given addition to several of the other plutonic rock types area, each mineral and each dating method employed associated with them. Although the apparent absolute should have a distinctively shaped age-retention curve ages of two plutons have been determined and the which depends on a number of factors, including size relative age relations between others are known, the and composition of the units and pressure-temperature complete sequence of intrusion has not yet been estab­ conditions. lished with certainty. In general, hornblende has retained almost all argon Potassium determinations were made in duplicate within a relatively short distance of the contact, on Baird and Instrumentation Laboratories flame whereas biotite has retained argon only at much greater photometers with a lithium internal standard. Argon distances. Muscovite is intermediate but is generally analyses were made on a 6-inch 60° Nier-type sector closer to biotite than to hornblende. Figure 17 diagrams mass spectrometer, using standard isotope dilution these qualitative observations, without regard to dis­ techniques. tances involved or size of intrusions. The expected K 40 decay constants: distribution of apparent ages 'for each mineral type in A.e==0.585 X 10-10 yr1 an older pluton can be obtained by projecting the curve A.e==4.72 X 10-10 yr1 onto a line extending outward from the contact with K40==1.19 X 10-2 atom percent the younger body (fig. 17). The areal distribution of Errors ( + values) have been assigned on the basis of apparent ages obtained by more than one method on both experience with duplicate analyses and uncertain­ several mineral types may initially lead to confusion. ties in the individual runs and represent 2u. The scatter of apparent ages can range from the true Early in the study of the plutonic rocks by the potas­ age of the host rock to the age of the younger pluton. sium-argon method, it was found that a pair of minerals Thus, in an area where isotopic ages of plutonic rocks from a single rock often gave strikingly different ages. show many discordances and apparent anomalies Efforts were then made to obtain rocks from which because of a younger plutonic event, interpretation is more than one mineral could be extracted for dating. simplified by initially comparing ages obtained using By analyzing mineral pairs, an age disturbance could only one dating method on a single mineral type. be detected if the two minerals yielded different ages. The unit which seems to best illustrate this qualita­ Of all the rocks dated, only two, the Starvation Flat tive treatment of the effects of a younger plutonic event . Quartz Monzonite and the Silver Point Quartz Mon­ is the Flowery Trail Granodiorite. Three biotite-horn­ zonite, appear to yield concordant ages on mineral blende pairs from this pluton show a progressive age pairs. loss in an easterly direction. The oldest age obtained Table 3 gives ages of the rocks analyzed, and figure for the pluton is 194 m.y. on hornblende from a sample 16 their locations. Exami:1.1ation of the two together (No.1, fig. 16) collected in the western part of the body shows that the amount of discordance between indi­ near the Jay Gould mine. Biotite from the same rock vidual mineral pairs within other plutons generally yields an age of only 98 m.y. Hornblende from another increases to the south and east. The discordances sug­ sample (No. 2) collected about 1.5 miles farther east gest a widespread thermal disturbance about 50 m.y. gives an age of 183 m.y., and biotite an age of 84 m.y. ago in the region just east of the report area. Although Sample 3, collected about 3.3 miles east of the first one, the Silver Point Quartz Monzonite lies in this direction yields ages of 143 m.y. for hornblende and 64 m.y. for and is among the youngest intrusive bodies in the area, biotite. · it is not known for certain if the thermal effects of this The apparent age of these three samples is compared pluton are widespread enough to have caused the dis­ with their location along an east-northeast-west-south­ cordances in the other plutons. If the Silver Point west line in figure 18. Although more data would be Quartz Monzonite is the source of the discordances, the desirable, the graph at least suggests an approach to POTASSIUM-ARGON AGES OF THE PLUTONIC ROCKS 53

TABLE 3.-Potassium-argon data on plutonic rocks

t uZ' s:~ Z' <..-. ·cGIGI .!I ~ utlG -;; Po '.-4 0~ .ae Mineral .. u r:l.GI sz. Pluton or rock type t! ~r:lo ! Location ~t ~0 Gl ~ e .e .... s ...s-- t ~-- << ~ 1 Flowery Trail Granodiorite...... Hornblende...... 1.44 4.348X 10-10 11.2 194±7 NWlf.i sec. 9, T. 32 N., R. 41 E. Biotite ...... 8.71 1.283X10-9 15.0 98±5 Do. 2 ...... do ...... Hornblende...... 1.41 4.007X 10-10 9.4 183±6 Center sec. 3, T. 32 N., R. 41 E. Biotite ...... 8.175 1.041Xl0-9 22.5 84±3 Do. 3 ...... do ...... Hornblende...... 1.39 3.042 X 10-10 11.4 143±5 NWlf.i sec. 1, T. 32 N., R. 41 E. Biotite ...... 8.825 8.481 X 10-10 69.9 64±3 Do. 4 Starvation Flat Quartz Monzonite.... Hornblende...... 6855 1.003 X 10-10 20.1 97±3 SW cor. SW% sec. 3, T. 34 N., R.40E. Biotite ...... 8.405 1.254Xl0-9 7.8 98±3 Do. 5 Phillips Lake· Granodiorite...... Muscovite ...... 10.765 1.092Xl0-9 24.0 67±2 NE% sec. 34, T. 34 N., R. 41 E. · Biotite ...... 9.360 7.922Xl0-10 10.0 56±2 .Do. 6 ...... do ...... Muscovite ...... 10.64 1.356Xl0-9 11.1 84±4 SW% sec. 8, T. 36 N., R. 42 E. (Colville 30-min quadrangle). Biotite ...... 9.26 1.089X10-9 9.5 79±3 Do. 7 ...... do ...... Muscovite ...... 10.665 9.244Xl0-10 9.1 58±2 NE% sec. 12, T. 33 N., R. 41 E. Biotite ...... 9.448 7.259X 10-10 11.4 52±2 Do. 8 Leucocratic dike ...... Muscovite ...... 10.72 1.366Xl0-9 11.1 84±2 NE% sec. 32, T. 35 N., R. 41 E. Biotite...... 7.895 6.996 X 10-10 45.4 59±2 Do. · 9 ...... do ...... Muscovite ...... 10.68 1.438X 10-9 13.8 89±6 SW% sec. 21, T. 35 N., R. 41 E. (Colville 30-min quadrangle). Biotite...... 8.695 9.568 X 10-10 29.8 74±2 Do. 10 Silver Point Quartz Monzonite...... Hornblende...... 817 7.304 X lO-ll 32.7 60±2 NW% sec. 11, T. 29 N., R. 41 E. Biotite ...... 8.70 6.502X 10-10 12.0 50±1 Do. 11 ...... do ...... Hornblende...... 6078 4.642X lO-ll 37.5 51±5 NW% sec. 5, T. 30 N., R. 44 E. (Newport 30-min quad.). Biotite ...... 8.45 6.016Xl0-10 21.8 48±1 Do. 12 Granodiorite ...... Hornblende...... 874 8.231 X lO-ll 26.6 63±2 NE% sec. 11, T. 29 N., R. 40 E. Biotite ...... 8.88 6.493X 10-10 10.8 49±2 Do. 13 Fine-grained quartz monzonite ...... Hornblende...... 338 1.745X1Q-10 17.51 320±23 NW% sec. 32, T. 30 N., R. 41 E. Biotite ...... 8.825 6.657 X 10-10 11.8 50±2 Do. 14 ...... do ...... Hornblende...... 3555 1.671 X 10-10 35.0 294±15 Do. 15 ...... do ...... Hornblende...... 374 1.136X 10-10 38.7 195±8 Do. Biotite ...... 8.455 6.445 X 10·10 48.3 51±2 Do. 16 Muscovite quartz monzonite...... Muscovite ...... 10.235 1.204Xl0-9 8.0 78±2 NW% sec. 25, T. 30 N., R. 41 E. 17 Two-mica quartz monzonite ...... Muscovite ...... 10.745 9.042~10·10 21.7 56±2 SE% sec. 9, T. 31 N., R. 42 E. Biotite ...... 9.12 7.568Xl0-10 14.4 55±2 . Do. 18 Aplite ...... Muscovite ...... 11.00 1.033Xl0-9 9.5 63±2 NE% sec. 34, T. 34 N., R. 41 E. finding the true age of the Flowery Trail Granodiorite. relation between higher apparent excess argon and The three biotites lie on a straight line which intersects lower potassium content, as might be expected in cases the 50-m.y. level approximately 2 miles east of sample of excess argon; in fact, the potassium values for these 3, while a smooth curve drawn through the three horn­ three samples are remarkably similar (table 3) . blende points intersects the 50-m.y. mark at about the Other plutons of early to middle Mesozoic age are same location. This construction is remarkably similar known from northeastern Washington and southern to the curves depicted by Hart ( 1964). The project~d British Columbia (Rinehart and Fox, 1972; Wanless point of intersection may be the point at which an out­ and others, 1965, p. 14), although, like the Flowery crop of the pluton causing the disturbance would be Trail Granodiorite, each is associated with internal age expected, or it may be the outer limit of complete argon discordances. Clark and Miller ( 1968) assigned the loss in the rocks that the pluton intrudes. Extrapolated Flowery Trail Granodiorite a Mesozoic(?) age on the westward, the hornblende curve appears to flatten out, basis of geologic relations. This assignment is here which may indicate that the "true age" is not much refined to Late Triassic or Early Jurassic. greater than the oldest age found for hornblende, 194 The next youngest pluton is the Starvation Flat m.y. Quartz Monzonite, which appears to be unaffected by These older hornblende ages do not appear to be due the younger intrusive rock, at least where the dated · to excess argon in low-potassium minerals, because the sample was collected. Hornblende from a specimen hornblendes have a relatively high potassium content, collected on the west border of the area yields an age averaging about 1.4 percent K20. Also, there is no cor- of 97 m.y.·, which agrees well with an age of 98 m.y. on 54 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

6(10 miles)

T. 35 N. EXPLANATION

~ ~ Silver Point Quartz Flowery Trail Granodiorite T. 34 N. Monzonite

Fine-grained quartz monzonite

Granodiorite Muscovite quartz monzonite

T. 33.N. Phillips Lake Granodiorite Two-mica quartz monzonite

Starvation Flat Quartz Monzonite Contact

T. 32 N.

T. 31 N. • ••• 0 ••••• 0 •••• 0 ••••

• • 0 •••• 0 ••• • • • 0 0. 0 ••••••

T. 30 N.

...... 0 .••••• . . . . .0 0 0 •••••• • • • • • 0. T. 29 N. • •••••••••••••• 0 0 .... 0 0 0 •••••••••••• 0. 0 • 0 ••• 0 • 4 8 MILES . • . • . • . • . • . • . • . • . • . • . • . • . • . • . • • . 0 .....0 ••••• . 0 • • • • • • • • • 0 ••••• 0 ••• 0 •• • • • • • • • • 0 0. 0 •••• 0. 0 •••

R. 40 E. R. 41 E. R. 42 E.

FIGURE 16.--Generalized map showing distribution of plutonic rocks in and around the Chewelah-Loon Lake area and the potassium-argon sample localities. Numbers correspond to sample numbers In table 3. POTASSIUM-ARGON AGES OF THE PLUTONIC ROCKS 55 w (..) Zt- 100 a:zW- W::::> 90 LLa: ~w 80 co w.J 70 ~0 <(> 60 LLID 00 ww ~~ 40 <(<( 1-1- 30 zw wa: 20 (..) a: w 10 Q. 0 Distance within an older plu­ ton from the contact with a younger pluton

50 70 80 90 Apparent ages of different minerals in map plan. Hornblende Shows graphically the relation between apparent ages and why at any given distance from a 10 20 30 40 50 60 70 80 90 younger intrusive body, different minerals give Muscovite different apparent ages

1 0 20 30 50 70 80 90 Biotite PERCENTAGE OF AGE DIFFERENCE RETAINED BY OLDER UNIT FIGURE 17.-Changes in apparent ages of hornblende, biotite, and muscovite in an older pluton intruded by a younger one. lips Lake Granodiorite collected within the area and West-southwest East-northeast one sample collected about 10 miles north of the area Vi 200 a: all yield discordant ages (table 9). Muscovite in the <( UJ northernmost sample (No.6) gives an age of 84 m.y., >- z 150 and biotite 79 m.y.; muscovite in the intermediate sam­ 0 ple (No.5), 15 miles south-southwest of sample 6, gives ::i ...J an age of 67 m.y., and biotite 56 m.y.; and muscovite ~ 100 in the southernmost sample (No. 7), 18 miles south­ UJ (.!) southwest of sample 6, gives an age of 58 m.y., and <( biotite 52 m.y. z1- UJ 50 Yates and Engels (1968, p. D245) reported similar a: <( but less pronounced discordances from probably cor­ 0... 0... relative rocks in the Deep Creek area at a locality about <( 0 2 3 20 miles north of the report area. A rock which is mod­ Sample number ally, texturally, and mineralogically identical with.the f-.-1 .5 1.8-.j Phillips Lake Granodiorite was mapped as part of the DISTANCE (MILES> Kaniksu batholith by Yates (1964) and gives a biotite FIGURE 18.-Apparent ages of hornblende and biotite from the age of 92+3 m.y. However, ~uscovite from a pegmatite Flowery Trail Granodiorite plotted against the distances sep­ associated with .this rock gives an age of 99+3 m.y arating specimen localities. (Yates and Engels, 1968). About 2 miles north of these biotite from the same specimen. The potassium-argon sample localities, the Spirit pluton, which Yates and dates allow revision of the Mesozoic(?) age assigned Engels regarded as probably part of the Kaniksu batho­ by Clark and Miller ( 1968) to Cretaceous. lith, gives a similar discordance (muscovite, 96+3 Muscovite-biotite pairs from two samples of the Phil- m.y.; hornblende, 94+3 m.y.; biotite, 91 +3 m.y.), and 56 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

biotite from a rock collected about 14 miles to the west gives a 100+3-m.y. age. They concluded that the bio­ 100 North-northwest ----:...-..- South-southeast tite in the eastern samples lost part of its argon and that en all these rocks were emplaced about 100 m.y. ago. If a: <( the correlation of the Phillips Lake Granodiorite with ~ 75 ' the Kaniksu rocks is correct, the probable age of the z 0 granodiorite is then 100 m.y., much older than any of _J the ages obtained so far for this rock in the report area. _J ~ 50 Two of the leucocratic dikes that are associated with w (!) the Phillips Lake Granodiorite and cut it in places were <( also dated. A sample of the southernmost of the two 1- z (No. 8) was collected on the west flank of Blacktail ~ 25 <( Mountain where the dikes intrude the Starvation Flat 0.. 0.. Quartz Monzonite. Biotite from this rock gives· a 59- <( m.y. age, and muscovite an 84-m.y. age. Sample 9, o~~--~------~~--~------which was also from a dike. intruding Starvation Flat 9 8 5 7 Quartz Monzonite, was collected about 2 miles north Sample number of sample 8 and yields a biotite age of 74 m.y. and a -+1 .5-.j-.-----6 2 .6--l muscovite age of 89 m.y. Since leucocratic dikes of DISTANCE - --. lowered by thermal events in many of the other rocks, ffi z 200 ·- -.::;------a:O the hornblende may give a misleading age for these . -- ...... -...... :3 100 three plutons. The granodiorite and the fine-grained 0.------.. --- ... ------_, - --

come apparent. 0.2 inch in size. Their average composition is An34, CENOZOIC HYPABYSSAL, VOLCANIC, AND SEDIMENTARY ROCKS 59

TABLE 4.-Mineral composition of selected mafic dikes [X=mineral present; A=mineral present, but altered]

Phenocrysts Groundmass til f T. R. ~ 9 No. Sec. (N.) (E.) o ·~ Remarks "lio -g ~ P-4 1...... 1 82 41 X A X X X X X X X Aphanitic groundmass. 2 ...... 29 84 41 X A X X X X X X X X X X X X X See mode (table, p. 5 8 ) • ~8 .....•...... 29 84 41 A X A X X X X X X X X X X X Color index over 40. · 4...... 9 82 41 X A X A X X X X X X X X X X Groundmass is high in potassium feldspar. 6 ...•.•.•.•.. 28 84 41 X X X X X X X X X X X Even-grained large hornblende phenocrysts.

6•••••••••••• 12 82 41 X X A X X X X X X X X X X Groundmass is high in potassium feldspar. 7...... 18 88 41 X X X X X X X X X X X X X 8...... 18 88 41 X X X X X X X X X X X X X X X See mode (table, p. 58 ). 9...... • 24 82 41 X X X X X X X X X X X X X X Do. 10....•...... 28 80 41 X X X X X X X X X X X X Gray aphanitic groundmass, pink potassium feldspar phenocrysts. 11...... 8 29 41 X X X X X X X X X X X X X X Low color index, large potassium feldspar phenocrysts. 12...... 8 29 41 X X X X X X X X X X X X X X X 18...... 8 29 41 X X X X X X X X X X X X X .X X 14...... 9 29 41 X X X X A X X X X X X X X X Same as 10 in appearance. 16 ...... 17 29 41 X X X X X X X XXXX XXX X Same as 11 in appearance, but no potassium feldspar phenocrysts.

16...... 1 29 40 X X A X X X X X X X X X X X Same as 11 in appearance. 17...... 82 80 41 X X X X A X X X X X X X X X X X Dark-gray even-grained phenocrysts are small. 18...... 16 29 41 X X X A X X X X X X X X X X Same as 11 in appearance, but no potassium feldspar phenocrysts. 19...... 1 29 40 X X A X X X X A X X X X X X X Rounded potassium feldspar phenocrysts. 20...... 6 29 41 X X A X X X X X X X X X X X Large potassium feldspar pheno­ crysts. Strong resemblance to Silver Point Quartz Monzonite.

21 ...... 29 80 41 X X X X X X X X X X X· X Dull-pink groundmass. 22 ...... 29 80 41 X A X X X X X X X X X X X X X Same as 17 in appearance. 28...... 6 81 42 X X High color index. 24...... 8 81 42 X A X I I I I I i X X X 26 ...... 16 81 41 X X X X X X X· X X X Dull-pink groundmass. 26 ...... 12 81 41 X A X A X X X X X X X X X X X Pink groundmass. 27 ...... 16 81 41 X A X A X X X X X X X X X X Dull-pink groundmass. 28 ...... 16 81 41 X A X A X X X X X X X X X X X · Do. 29...... 8 81 42 A X X X X X X X X Same as 23 in appearance.

1Probably includes ilmenite. ~Margin of same dike as No.2. whereas average groundmass composition is An15• In unusually mafic dike about 2 miles west of Phillips addition, potassium feldspar phenocrysts as much as Lake. The borders of all pyroxene crystals examined 0.8 inch long are found in dikes in and around the Silver are altered to hornblende, biotite, or chlorite. Point Quartz Monzonite that may be related to that Groundmass grains are euhedral to anhedral and pluton. Hornblende, the most common mafic pheno­ range from 0.001 to 0.005 inch in size. The variation in cryst, is found in about 90 percent of the dikes. Biotite grain size is somewhat a function of the proximity to is found in about 80 percent, but in many of these it is the borders, although chilled margins are not obvious altered partly or completely to chlorite. In dikes that in the field. Width of dike and grain size of groundmass contain phenocrysts of bo.th biotite and hornblende, are generally related. one of the two minerals is always altered. In most, only The groundmass is some shade of gray in all dikes one of the mafic minerals is found in the groundmass, except the two large ones south of Cottonwood Creek. and it corresponds to the unaltered mafic phenocryst. The darker dikes contain a higher concentration of Almost all the biotite in the dikes associated with the mafic minerals in the groundmass than do the lighter Silver Point Quartz Monzonite is severely altered. ones. The color of the dikes south of Cottonwood Creek These crystals may not have been stable in the physical­ is a distinctive dull pink, which might be the result of an chemical conditions which produced the potassium abnormally large amount of small potassium crystals feldspar phenocrysts found in all these rocks. Pyroxene in the groundmass. As with the feldspar-rich dikes phenocrysts are found only in the large dikes south of associated with the Silver Point Quartz Monzonite, Cottonwood Creek and in the chilled margins of an the biotite in all the pink rocks is highly altered. 60 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

The dikes are younger than any of the plutonic rocks. Chemical analysis, CIPW norm, and mode of andesite southwest of Chewelah Engels used potassium-argon methods to date several [Analysts: P. L. D. Elmore, Hezekiah Smith, James Kelsey, Lowell Artis, dikes in the northeast quarter of the Colville 30-minute and James Glenn] Chemical analysis· CIPWnorm Mode quadrangle (Yates, 1964). Two lamprophyre dikes (weight percent) (weight percent) ( 11olume percent) which are similar to some of the mafic dikes in the Si02 ...... 58.3 Q ·········· 8.0 Plagioclase ...... 8 report area and may be related to them were dated as AhOa ...... 14.7 or ...... 19.0 Hornblende ...... 20 49.9+ 1.5 m.y. and 51.7 + 1.5 m.y. (Yates and Engels, Fe:aOs ...... 2.5 ab ...... 25.6 Pyroxene ...... 3 1968, p. D243). FeO ...... 3.8 an ...... 17.0 Olivine ...... 1 MgO ...... 5.8 wo ...... 3.3 Biotite ...... 1 ANDESITE CaO ...... 5.8 en ...... 14.5 Groundmass ...... 67 N&O ...... 3.0 fs ...... 3.7 Relatively well exposed black andesite underlies an K,O ...... 3.2 mt ...... 3.6 100 area of about a quarter of a square mile, 3 miles south­ H20+ ...... 1.1 il ·········· 1.7 west .of Chewelah. It is not found any other place in the TiO, ...... 89 ap ...... 1.2 report area, but somewhat similar rocks have been P20s· ...... 51 cc ...... 2 mapped in the Turtle Lake quadrangle to the south­ MnO ...... 11 co, ...... 10 99.7 west by Becraft and Weis ( 1963). They referred these rocks to the Gerome Andesite (Weaver,. 1920) and 99.61 assigned them to the Oligocene on the basis of plant birefringence. The olivine has a 2Vx of about 80°-90°. fossils in interbedded tuffaceous rocks (Becraft and Biotite is much less abundant than the hornblende but Weis, 1963, p. 37). The Gerome Andesite has also been is surrounded by the same rims of fine-grained opaque mapped in the west half of the Hunters quadrangle, just minerals. Its pleochroism is X==light yellow brown and 22 miles west of Valley, by Campbell and Raup ( 1964), Y==Z==red brown. and in the adjacent Wilmont Creek quadrangle by Pyroxene is the second most abundant mafic mineral Becraft (1966). Becraft renamed the unit the Gerome but is not easily identifiable in hand specimen. It ranges Volcanics. in size from about 0.12 inch down to microlites in the The lithologic descriptions given by Becraft and groundmass and is the only mafic mineral that appears W eis partly fit the andesite in the report area, but they to be relatively free of any alteration. The 2Vz is about differ in some important aspects. The andesite in the 45°-50° in some crystals, but about 0°-25° in others. Turtle Lake quadrangle is higher in Si02 and Al20a but At least some of the pyroxene appears to be pigeonite, is noticeably lower in FeO and MgO than the andesite and some augite. Plagioclase ranges in size from about described here. Other constituents are about the same.. 0.12 inch- down to microlites in the groundmass. It The andesite appears to rest on the Stensgar Dolo­ appears to be andesine but is highly zoned. mite, but the exposures do not show whether the con­ The finer grained plagioclase and pyroxene, along tact is extrusive or intrusive. The top is not preserved, with opaque minerals and partially devitrified brown and no flow structures were observed. If the andesite glass, form an aphanitic groundmass with a pilotaxitic is extrusive, and assumed to be near horizontal, it must texture. Much of the very fine grained material may be be at least 240 feet thick. devitrified glass. Although it is too fine grained to Phenocrysts of olivine, hornblende, and, to a lesser identify in thin section and has not been X-rayed, its degree, biotite are easily seen in hand specimen. Plagio­ alkali content (see preceding table) suggests that much clase and pyroxene crystals larger than the groundmass, of it is quartz, potassium feldspar, and albite. but smaller than the phenocrysts, are abundant al­ though obvious only in thin section. The mode in the BASALT table that follows is representative of the andesite. Hornblende is the most abundant mafic mineral. Al­ About 10 square miles in the southwest part of the most all of it is highly oxidized and has thick rims of area is underlain by basalt, but more than half of this magnetite or ilmenite finely disseminated in a semi­ is masked by younger glacial and alluvial debris. The opaque material. Inside the alteration rim the remain­ basalt appears to be confined chiefly to elevations below ing hornblende is slightly altered and highly zoned. It 2,500 feet, .except on the northwest slope of Jumpo:ff is probably basaltic hornblende because of the oxida­ Joe Mountain, where it is found as high as 2,900 feet. tion products, medium to high birefringence, and pleo­ Individual flows and flow thicknesses could not be dis­ chromism, X==light tan, Y==yellow brown, and Z== cerned, and so the attitude of the flows could not be deep golden brown. Olivine crystals are as much as 0.08 measured directly. Since the base is found at about the inch in size and are surrounded by fine-grained olive­ same elevation in most places in the area, the flows must green to golden alteration products that have moderate be nearly horizontal, except around irregularities in the CENOZOIC HYPABYSSAL, VOLCANIC, AND SEDIMENTARY ROCKS 61 underlying surface. In the NE% sec. 30, T. 31 N., R. 41 The formation was originally named by Pardee and E., Empire Explorations Inc. drilled a test well which Bryan (1926, p. 1), who thought that the unit was penetrated 139 feet of basalt. Assuming that the basalt overlain by the Columbia River Basalt. Kirkham and is nearly horizontal, this is the most accurate measure­ Johnson (1929, p. 483) showed that the Latah Forma­ ment of a partial section but probably does not repre­ tion actually interfingers with the basalt. The contact sent a maximum thickness in the area. l;>etween the basalt and Latah Formation is not exposed The rock is black to dark gray, non porphyritic, and in the report area. locally vesicular. Columnar jointing is well developed in In the two claypits the formation consists of sand­ places but is not obvious because exposures are poor. stone, siltstone, silty clay, and what Glover (1941, p. Petrographically, the rock has a hyalo-ophitic texture. 282) described as "bog iron." The so-called bog iron Plagioclase, pyroxene, and olivine crystals generally consists of rust-colored cohesive material which re­ constitute about 45 percent of the rock. Dark-brown sembles limonite and which cements clastic material almost opaque glass with abundant disseminated mag­ ranging in grain size from coarse sand to clay. The sur­ netite and (or) ilmenite makes up the rest of the rock. face of the bog iron is botryoidal in many places. The Several thin sections of rocks from different localities bog iron may be confined to a single bed at both pits showed little variation in mineralogy or texture. The and if so may prove an aid to prospecting for clay. (See only notable variation was found in a specimen collected Miller, 1969, p. 6 and fig. 3.) The clay is unconform­ in the NW% sec. 17, T. 31 N., R. 41 E., which contained ably overlain by glacial material. about a third more plagioclase and pyroxene than other specimens and about a third less glass and olivine. The CONGLOMERATE following table gives the modes, in volume percent, for Small isolated patches of well-indurated unsorted specimens of this basalt: pebble to boulder conglomerate are found on the hill

Spocimona from throe One specimen from sec. southwest of Chewelah and on the south and east flanks localities1 17, T. 91 N., R . .u E. of Cliff Ridge. At both localities, the clasts are com­ Plagioclase ...... 26 35 posed of the unit on which the conglomerate is lying or Pyroxene ...... 13 17 Olivine ...... 6 3 of nearby lithologies. Glass ...... 45 35 Southwest of Chewelah, the conglomerate appears to Opaque minerals ...... 10 10 cling to the side of the hill. Stratification is not obvious Total ...... 100 100 in the rock, and so it is not known if this is a buttress 1Averagc of th1·cc modes; less than 3 percent variation in any constituent. relationship. At Cliff Ridge, the conglomerate is uncon­ Weaver (1920, p. 99) and Jones (1929, p. 50) applied formably overlain· by glacial material. Many of the the name Camas Basalt to all the Tertiary volcanic clasts in the conglomerate southwest of Chewelah are rocks in the report area but recognized that the basalt fairly well rounded, but most of those in the unit at is largely continuous with the Columbia River basalts Cliff Ridge are angular. to the south. They erroneously included the andesite 3 The age of the rock is not known. The conglomerate miles southwest of Chewelah with the basalt, however. ·a:t one locality may not even be the same age as at the Griggs (1966) mapped basalt of the Columbia River other. The unconformity at the base of the conglomer­ Group within 2 miles of the southwest comer of the ate and the fact that the rock is well indurated lead us report area. Becraft and Weis (1963, p. 39--40) reported to suspect a Tertiary age. ' at least 900 feet of Columbia River Basalt in the Turtle GLACIAL, ALLUVIAL, AND TALUS Lake quadrangle, at least part of which appears to be DEPOSITS, UNDIFFERENTIATED "Late Yakima flows." D. A. Swanson, of the U.S. Geo­ logical Survey, examined several thin sections of basalt Glacial, alluvial, and talus deposits were mapped as from the report area and reported that they strongly one unit. Alluvial material is confined to the beds and resemble Yakima Basalt, which is of Miocene and Plio­ flood plains of modem streams. Glacial debris consists cene age (oral commun., 1966). chiefly of sand and gravel and locally fine-grained lacustrine deposits. The debris forms moraines, out­ LATAH FORMATION wash plains, terraces, and thin mantles on hillsides. Hosterman (1969, p. 56) assigned the clay-bearing The distribution of glacial erratics, striae, and strata found in two small claypits in sec. 4, T. 29 N., R. moraines suggests that ice covered the western two­ 42 E., and sec. 32, T. 30 N., R. 42 E., to the Latah For­ thirds of the report area and that a lobe moved east­ mation of Miocene age. In an earlier report, Miller ward from the main body in Burnt Valley. The presence (1969, p. 4) erroneously included these clay beds in the of glacial debris on top of Quartzite Mountain led Clark Quaternary. and ¥iller (1968, p. 3) to surmise that the glacial ice 62 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON in the Colville River valley must have been more than tation is so great and the structure so complex, the 2,000 feet thick. Chewelah Mountain does not appear authors have tried their best to distinguish observed to be glaciated above the 5,000-foot elevation, although from interpreted structures in the following section bedrock is deeply frost riven. Similarly, that part of and, as much as possible, to point out the basis for Calispell Peak above this elevation must have pro­ interpretations. truded above the continental glaciers, but two cirques Two distinct structural blocks are evident in the on the north side of Calispell Peak indicate local moun­ report . area. (See fig. 8) The eastern block, which tain glaciation, presumably during the waning phase underlies most of the area, consists largely of Belt rocks of glaciation. that form the west limb of a large anticline. Numerous Moraines are common in the area, but many have faults cut this fold, and the northern part of the fold been eroded almost beyond recognition. A· prominent is overturned toward the west. The western block is moraine is responsible for the sharp switchback in the underlain primarily by the Deer Trail Group, which road in the NE~ sec. 24, T. 33 N., R. 41 E. Additional here forms the north end of the magnesite belt. Signifi­ small moraines are found at about the 3,600-foot level cant differences in stratigraphy and structures within on the slopes north of Burnt Valley. the blocks suggest that a major fault separates them The erosionally resistant north-south belt of Addy and that a large but unknown amount of lateral move­ Quartzite~ Striped Peak Formation, and upper Wallace ment has occurred. The J umpoff Joe fault may be this Formation appears to have dammed westward-flowing fault, but this possibly cannot be demonstrated con­ glacial streams and caused deposition of large volumes clusively. (See section "Structure Separating the Two of glacial debris in basins east of it. The largest of these Blocks.") At the few localities where the fault is ex­ basins is now drained by Cottonwood Creek. Others posed, it appears to dip shallowly to the west and to the north are now drained by Sherwood Creek, places Precambrian Deer Trail rocks on the west side Thomason Creek, South Fork of Chewelah Creek, against Paleozoic rocks on the east side. Most of the North Fork of Chewelah Creek, and Bear Creek. fault, however, is concealed beneath the surficial debris Poorly indurated lacustrine materials, probably de­ filling the Colville Valley, and so it is not known for posited in ponded water behind ice dams or moraines, certain whether the various segments mapped as the are found at several places in the area. An excellent Jumpoff Joe fault are in fact parts of a single, con­ example is exposed in small roadcuts at the top of the tinuous fault. divide which lies on the line between sees. 15 and 16, STRUCTURES IN THE T. 31 N., R. 41 E. The sediments there are fine grained, BELT SUPERGROUP BLOCK light gray to white, and thin bedded. Almost none of the lacustrine deposits crop out in natural exposures. FOLDS Although they contain some clay minerals, these depos­ The northerly trending west limb of a large anticline its do not have the high proportion of clay minerals is evident from the outcrop pattern and attitudes of found in the lake sediments previously described by Belt rocks in the east-central part. of the report area. Miller (1969, p. 4). (See pis. 1, 2; figs. 8, 22). Southeast of Deer Lake the Talus deposits are found chiefly below bold outcrops, ·1ower part of the Belt Supergroup strikes northwest formed mainly by the quartzite units. and dips at moderate angles to the west. North of Deer Lake the strike of the section swings progressively more STRUCTURE northward, and the dip becomes increasingly steeper. Structural and stratigraphic interpretations are At about the latitude of Grouse Creek, the section closely interrelated in the report area because interpre­ strikes north-northeast and is overturned towards the tations involving one are almost invariably based on west. Northward it becomes progressively more over­ the other. Much of the structure shown on the map and turned, but the strike remains fairly constant up to the cross sections is inferred or at best based in part on latitude of Quartzite Mountain. There, it begins ·to indirect evidence because of the generally poor expo­ swing farther eastward until in the vicinity of God­ sure. This is particularly true for much of western part dards Peak the section strikes east-northeast, and of the area, where the structural complexity is greatest. locally east-west, and is then cut off by plutonic rocks. To a certain extent, the degree of interpretive freedom The east limb of this anticline, east of the report is inversely proportional to the amount and quality of area, is not nearly as well defined. Attitudes of the Belt exposure. During the mapping, as stratigraphic infor­ rocks between Fan and Horseshoe Lakes (fig. 2), about mation increased and interpretations became more re­ 7 miles east of Deer Lake, indicate that the rocks are fined, the complexity of the structural history became near the axis of the fold, as the strike swings to east­ increasingly apparent. Because the amount of interpre- west, and in places east-northeast. In this area all the STRUCTURE 63 strata are right-side-up and dip gently to the south. If single structure. About 5 miles west of them, he mapped the strike of the section continues to swing to the north­ an anticline which Schroeder ( 1952, p. 26) had previ­ east, the strata will form the east limb of the anticline. ously named the Snow Valley anticline. This anticline However, plutonic rocks may interrupt this continua­ is en echelon with an unnamed anticline that Barnes tion. mapped near the southwest end of Priest Lake, but is This anticline, here named the Nelson Peak anticline, separated from it by about 8 miles of intervening and the partially exposed syncline to the west, here younger granodiorite. All the folds are rather open, and named the Chewelah syncline, are shown in figure 22. the strike of the axes varies somewhat. The general They are the westernmost in a series of rather evenly strike, however, is about due north. spaced generally north-south-trending folds extending In the Bead Lake area, Schroeder (1952, p. 25) map­ eastward almost to Pend Oreille Lake in Idaho. Barnes ped only the east limb of a fold that he called the New­ ( 1965, pl. 1) mapped two synclines, the Priest River port syncline and suggested that its axis lay to the and the Peewee, in the Priest River valley in Idaho. The west, approximately in the position of the Pend Oreille two folds project toward one another and may' be a River. From the strike of the beds of the east limb, the

1 18°00' 117° 30' 117° 00'

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Note: Paleozoic and Precambrian rocks in this area are covered EXPLANATION by surficial material or intrud­ FOLDS ed by plutonic rocks --+--?- --n-·-?- Normal Overturned Anticline -t--+- u ?- Normal Overturned Syncline Showing trace of axial plane and direc­ tion of dip of limbs. Dashed and queried where inferred

0 10 20 MILES

FIGURE 22.-Location of major fold axes in the Belt Supergroup block. " 64 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON axis should have about the same strike as the Snow Valley anticline. The Priest River syncline, Snow Valley anticline, and Newport syncline are all rather open folds with gentle to moderately dipping flanks. The Nelson Peak anti­ cline and Chewelah syncline are noticeably tighter than these three and are in part strongly overturned. There is an equally noticeable increase in the intensity of folding from south to north within the report area. Both limbs of the Nelson Peak anticline at about the latitude of Deer Lake are normal, but to the north, at the lati­ tude of Chewelah Mountain, the west limb is strongly overturned to the west. The Chewelah syncline, al­ though ill defined and complicated by faulting in the • southern part of the area, is reasonably well developed Valley• VaUey ~ from the Jay Gould Ridge area north to Johnson Mountain. Comparison of cross sections B-B' and A-A' (pl. 1) shows the change between these two areas. Although strongly overturned, the syncline at the lati­ tude of B-B' is rather regular. Along A-A' however, the limbs of the fold are attenuated, and the nose is con­ A B c siderably thickened. The structural change in thickness 2 4 6 MILES is due at least in part to the extreme development of a 0 closely spaced slip cleavage in this area. The cleavage APPROXIMATE SCALE appears to be about parallel to the axial plane of the FIGURE 23.-Sketch maps showing alternate interpretations of fold and is so pervasive that·it has given the rocks a the relation of the Addy Quartzite to the northwest-trending faults between Eagle Mountain and Jumpoff Joe Mountain. highly sheared appearance. In the vicinity of B-B' the A, Present mapped relation. B, Lower contact of Addy slip cleavage is relatively low dipping and roughly par­ Quartzite restored to position undisturbed by faults. Inferred allel to the axial plane of the syncline but is not nearly northeast-striking fault has been removed. C, Probable con­ as well developed as it is to the north. The right-side-up figuration of lower contact of Addy Quartzite if movement on part of the syncline is exposed on Johnson Mountain, northwest faults had all been post-Addy. Arrows indicate in Ten Mile Creek southeast of the Flowery Trail apparent offsets only. Granodiorite, and in the vicinity of Deer Lake. by a north-northeast-trending, perhaps roughly con­ temporaneous, fault before they reach the quartzite. FAULTS Fault 5, which has a minimum apparent offset of about A group of northwest-trending faults offset the Belt 11,500 feet, passes through an area covered by glacial Supergroup rocks with rather large apparent displace­ drift, across which the Addy Quartzite has a roughly ment, all but one in a left-lateral sense. Parts of the comparable offset. The fault relations in this covered traces of all of them are masked by glacial debris, and area are largely interpretive and are complicated by so their position can only be inferred in many places. a well-documented large northeast-trending fault and The apparent movement along these faults could have a small thrust fault, the existence of which is based been strike slip or dip slip, probably the latter, and is largely on interpretation. Most of the apparent left­ thought to be Precambrian in age. These faults are con­ lateral offset of the Addy Quartzite in this area is prob­ sidered to predate the development of the major folds ably caused by dip slip on the large northeast-trending in the Belt Series, as they do not appear to cut the fault just mentioned. This fault clearly offsets the lower Cambrian Addy Quartzite, which is involved in the contact of the Addy Quartzite and also offsets the in­ folding (fig. 23). The fault east of Quartzite Mountain ferred contact between the quartzite and overlying car­ (No.2, pl. 1) has a minimum apparent displacement of bonate rocks south of where it cuts fault 5. 10,000 feet in an apparent left-lateral sense. At the The trace of fault 6 is covered for its entire length, west end the displacement of the lower contact of the but the existence of the fault is inferred in order to Addy Quartzite is only about 1,000 feet and is inter­ explain the juxtaposition of unlike Belt units to the preted as renewed post-Cambrian movement along a north and south of it. Because of the dips of the strata Precambrian fault. Faults 3 and 4 do not cut the Cam­ involved, a prohibitively large dip slip would be required brian quartzite, although they appear to be truncated to bring the rocks here from their original location. STRUCTURE 65 Although broken by other faults, fault 6 appears to tures are exposed well enough that short segments of a strike northwest. The best reconstruction of premove­ fault can be drawn. Most of the segments whose strike ment paleogeology necessitates about 7 miles of right­ can be determined with some reliability trend north­ lateral slip along this fault. However, this distance may west. Gross offsets such as those east of Quartzite bear little resemblance to the actual amount of move­ Mountain and in the vicinity of Cottonwood Creek are ment along the fault because of complications by other obvious, but actual fault traces could be quite irregular faults in the immediate area and uncertainties in cor­ there as exposure is poor even in areas shown as bedrock. relation of offset features across the fault. The best To minimize interpretive prejudice, separated segments "match" across the fault appears to be the syncline on along a fault have been connected by relatively straight Deer Lake Mountain with· a similar syncline on the lines through areas of sparse outcrop or glacial cover, south flank of Blue Grouse Mountain. Both folds have even though the trace may actually be quite irregular. the same axial strike, are rather open and involve the Previously published cross sections (Miller, 1969) same stratigraphic units. If they are offset parts of the show the faults unfolded. Neither interpretation can be same structure and are related to the Nelson Peak anti­ conclusively proved or disproved, but as the Addy cline and Chewelah syncline, then fault 6 must have Quartzite is involved in the folding, the faults, which formed after the folding and is much younger .than apparently predate the quartzite, also must be folded. faults 1 through 5 are considered to be. Differential movement along preexisting faults dur­ Faults with approximately the same northwest strike ing folding, in addition to the later small-scale thrust­ as fault 6, and with right-lateral offset as well, are ing, has created a locally complicated structural knot abundant and well documented in the Coeur d'Alene where the Cottonwood Road intersects the Addy district and in the Clark Fork quadrangle. The two Quartzite (sec. 5, T. 31 N., R. 41 E.). Much of the .rock largest and most widely known of these are the Hope in this small area is highly crushed, broken, or tightly fault in the Clark Fork and Libby areas, and the folded. If exposures were more complete, detailed study Osburn fault in the Coeur d'Alene district. Harrison here could help to better define the relations between and Jobin ( 1963, p. K28) reported 16 miles of apparent the various structures. right-lateral offset on the Hope fault but estimated that some of this is due to vertical movements. They STRUCTURES IN THE suggested that the major strike-slip movement took DEER TRAIL GROUP BLOCK place in the Precambrian and that the dip-slip move­ The Deer Trail Group block and most of the struc­ ment occurred in Laramide or ·post-Laramide to pre­ tures within it strike almost unvaryingly northeast­ Pleistocene time. southwest. Beds, faults, and, to a lesser degree, folds Hobbs and his colleagues estimated as much as 16 diverge little from this trend. The trend is not well dis­ miles of right-lateral strike-slip movement along one played in the relatively limited outcrops of the Deer segment of the Osburn fault. Detailed structural analy­ Trail Group in the report area but is excellently devel­ sis suggests that the major strike-slip movements along oped southwest and northeast of the area. North-south this fault occurred after the intrusion of the 100-m.y.­ and northwest-southeast trends within the Belt Super­ old monzonitic stocks in the vicinity but before the group block appear to be truncated by the Deer Trail extrusion of the Miocene Columbia River basalts Group. The differences in trend and the truncation (Hobbs and others, 1965, pl. 10, p. 128). Even though distinguish one block from the other. No large struc­ the major movement along this fault is probably Cre­ tures in the Deer Trail Group block strike northwest­ taceous, a zone of weakness approximately coincidental southeast. A few faults strike northwest-southeast, but with the present position of the fault may have existed none have large displacements or great extent (Camp­ there much earlier (Hobbs and others, 1965). bell and Loofbourow, 1962;pl. 1). The probable strike-slip movement along fault 6 in The rocks north of Chewelah form the northeast the report area may have occurred during the Creta­ end of the magnesite belt. Most of the rocks here are ceous, when the Osburn fault was most active. covered by glacial and alluvial material, but the north­ If the northwest-trending faults are folded, as the east-southwest trends that characterize the magnesite cross sections indicate, their trace on plates 1 and 2 belt are better developed here than elsewhere in the should be sinuous rather than straight, especially report area. Because exposure is so sparse in this part where they pass from a slightly overturned to a more of the report area, a small amount of reconnaissance tightly overturned part of the section and where they mapping was done just outside the west boundary, in cross areas of rugged topography. The faults probably the Iron Mountains, where the rocks are fairly well do have sinuous trace, but exposure is simply not ade­ exposed. Structure and stratigraphy can be projected quate to map actual fault traces. Locally, offset fea- from there into the report area. A series of four north- 66 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON east-southwest-trending moderate-angle reverse faults, however, the difference in dynamic metamorphism be­ which project into the area, are well developed in the tween the two sections is real and noticeable. Iron Mountains. Two more, not well defined in the Iron In addition to structural and stratigraphic contrasts, Mountains, are found in the report area. Although they a significantly different group of rocks overlie the Deer appear to be steeper, almost all the faults that Camp­ Trail Group and Belt Supergroup blocks. The Addy bell and Loofbourow (1962, pl. 1) mapped in the mag­ Quartzite rests on the Huckleberry and Monk Forma­ nesite belt are reverse faults and have a similar strike tions in the Deer Trail Group block, whereas it directly and trend. overlies the Belt Supergroup in the other block. In the north half of the magnesite belt, Campbell and There are only two localities in the report area where Loofbourow showed an inferred nqrthwest-southeast the H:uckleberry or Monk Formations possibly overlie fault folded by a locally developed north-south-striking Belt rocks. In sec. 7, T. 33 N., R. 41 E., and in sec. 29, anticline and syncline. Another anticline and another T. 34 N., R. 41 E., small areas are underlain by green­ syncline with the same axial strike are found on Gold stone and amphibolite injected by many leucocratic Hill and Deer Mountain 3 miles northwest of Chewelah. dikes. These localities are just east of what may be the There the folds appear to involve the Addy Quartzite main structure separating the Deer Trail and Belt and are therefore post-Cambrian in age. Campbell and blocks at this latitude. However, another fault which Loofbourow suggest the sequence of structural events also may be the main break passes just east of the in the magnesite belt began with development of the greenstone and amphibolite. northeast-southwest faults, followed by local folding Across the international boundary, and on strike with subsequent development of the few northwest­ with the Priest River Group, Rice (1941, map 603A) southeast faults. Presumably the northeast-southwest showed a thick section of the Purcell Series (the Can­ strike of the entire magnesite belt was established dur­ adian correlative of the Belt Supergroup) overlain by ing or before development of the faults with that strike. the Irene Volcanics and Toby Conglomerate (the Can­ adian correlative of the Huckleberry Formation). Al­ OTHER DIFFERENCES though the descriptions by Rice (1941, p. 8-13) fit the BETWEEN THE TWO BLOCKS Belt equivalent Purcell Series much better than the Priest River Group, there is some doubt as to whether The primary lithologic differences between the Deer the specific section in the southwest corner of the map Trail Group and the upper part of the Belt Supergroup area fits the descriptions. Furthermore, it is not known are exaggerated by differences in degree of dynamic if the structure separating the Deer Trail and Belt metamorphism. Throughout most of the magnesite blocks in the report area extends this far north. belt, the argillites of the Deer Trail Group are to some Along its strike length the Huckleberry Formation is degree phyllitic and (or) slaty. This is especially true intermittently continuous for at least 90 miles. Near of the Togo Formation and the McHale Slate. The the center of the magnesite belt, it is 4,500 feet thick Edna Dolomite contains argillite zones as much as 600 (Campbell and Loofbourow, 1962, p. F22, F23). Only feet thick and also bedding-plane partings of argillite. 10 miles east, however, across the strike of the magne­ Almost all the argillite is phyllitic; in places it even . site belt, it is not found between the Cambrian and imparts a phyllitic look to the dolomite. From recon­ Belt rocks. The basin in which the Huckleberry Forma­ naissance in the magnesite belt, the authors gained tion and its equivalents were deposited may, therefore, the impression that the Deer Trail Group is noticeably have been elongate in a northeast-southwest direction. and consistently more phyllitic than the Belt Super­ Even so, a basin this long might well have greater lateral group east of the Colville Valley. Deer Trail Group extent, and the narrow configuration preserved today rocks are relatively undeformed only in isolated pockets may be the result of structural shortening across the -for example, the Stensgar Dolomite 4 miles north­ basin. Erosion prior to the deposition of the Addy west of Chewelah on the north side of U.S. 395 and Quartzite is known to have removed parts of both Pre­ the argillite in the SW cor. sec. 22, T. 33 N., R. 40 E. cambrian sections and could have removed all or most (fig. 6). The McHale Slate within 20 feet of the Stens­ of the Huckleberry and Monk Formations from above gar Dolomite at the locality just mentioned, however, the Belt Supergroup now preserved east of the J umpoff is converted almost to a phyllonite, as is argillite within Joe fault. 100 feet of the undeformed argillite in sec. 22. In con­ There is a consistent difference in trend between the trast, the Belt Supergroup rocks east of the Colville Deer Trail Group and the Belt Supergroup block. North Valley are only locally phyllitic, and where they are of Chewelah the northeast-trending Deer Trail rocks it is due to the extreme development of a slip cleavage, abut younger plutonic rocks but continue with the particularly in the northern part of the area. In general, same trend on the other side of the batholith, in the STRUCTURE 67

Metaline quadrangle. In the Metaline quadrangle they fault is steeper on Eagle Mountain but may be slightly are called the Priest River Group (Park and Cannon,, downdropped by a younger, high-angle fault. North of 1943, p. 6), but are correlated by Becraft and Weis Eagle Mountain relations are obscured by glacial cover. (1963, p. 16) with the Deer Trail Group. The fault appears to bend to the west at the north Just south of the Metaline quadrangle, in the Bead end of Eagle Mountain and then continue northward Lake area, Schroeder (1952, pl.1) showed the Newport through The Tinderbox. The rocks in that area .are Group (Belt Supergroup) with a generally north-south extremely deformed and resemble those adjacent to the strike. Although the Newport and Priest River Groups fault on Eagle Mountain. From The Tinderbox the are separated over a distance of 15-20 miles by younger fault must follow Bayley Creek for a short distance and plutonic rocks, their structural relationship appears to then continue to the northeast, forming the boundary be the same as that of the Belt Supergroup and Deer between the Starvation Flat Quartz Monzonite and the Trail Group, respectively, in the report area. In both Phillips Lake Granodiorite. Where it cuts through the areas the Deer Trail Group or its correlatives are con­ plutonic rocks, the fault forms a prominent mylonite fined to the northeast-trending block, which appears to and cataclastic zone as much as· 500 feet wide. How­ truncate the less well defined north-south trend of the ever, from the south boundary of sec. 31, T. 34 N., R. Belt Supergroup and its correlatives. Only ·locally in 41 E., to the north edge of the area the identity of this the northeastern part of Washington are the Belt rocks fault is uncertain. It may be the Jumpoff Joe fault, one known to have this northeast regional trend, and only of the northeast-striking faults which project from the locally does the Deer Trail Group diverge from it. southwest, or a combination of both. The mylonite and cataclastic zone is thought to be part of the Jumpoff STRUCTURE SEPARATING TilE TWO BLOCKS Joe fault because it dips from 30° to 45° NW. In addi­ tion, no extension of the J umpoff Joe fault is known on . Stratigraphic differences between the Belt Super­ the northwest side of the zone. group and Deer Trail Group, structural differences be­ An inferred fault passes just east of The Tinderbox, tween the two blocks to which they are confined, and across the north fork of Chewelah Creek, and up Bear differenees in the sections which overlie the two lead to Canyon (pl. 1). It merges with the Jumpoff Joe fault to the conclusion that they are separated by a thrust fault the south and with the mylonite and cataclastic zone along which an undetermined, but possibly large, to the north. The fault is drawn through a number of amount of movement has occurred. This fault is isolated but extremely contorted and brecciated out­ thought to be the Jumpoff Joe fault. However, subse­ crops and may represent the main trace of the J umpoff quent faulting, possibly· regional tilting, and conceal­ Joe fault. ment by glacial and alluvial debris combine to make If the Jumpoff Joe fault is the major structure that study of the fault contact difficult. juxtaposes the two Precambrian sections, then the The Jumpoff Joe fault is exposed at several localities Deer Trail Group has been thrust over the Belt Super­ in the report area. It separates rocks of the Deer Trail group; but if it merely downdrops the major structure, Group from those of the Belt Supergroup, but whether then, because of the distribution of the two sections, it juxtaposes the two sections or merely downdrops the the Belt Supergroup would -be thrust over the Deer major structure is uncertain. The fault is well located Trail Group (fig. 24). Although the field relations per­ but poorly exposed on the hill immediately west of mit either conclusion, the former is favored because it Jumpoff Joe Lake; here Paleozoic carbonate rocks dip is simpler and because the trends of the Deer Trail under argillite probably belonging to the Deer Trail block appear to truncate those of the Belt Supergroup Group. South of there the fault is covered by younger block. The fact that it is not even known with certainty deposits for a distance of 2 miles but crops out on the which section has overidden which, or for that matter if hill north of Springdale, where it again places presumed one section is even thrust over the other, illustrates how Deer Trail argillite against Paleozoic carbonate rock. equivocal the data concerning this problem are. Re­ Here the fault dips at a moderate to shallow angle gardless of the exact form of the structure, however, under the Deer Trail Group and is clearly a low-angle the obvious differences in trend, internal structures, reverse or thrust fault. The fault is covered from Jump­ and stratigraphy of the two blocks indicate the exist­ off Joe Lake to Chewelah. It is exposed on Embry Hill ence of some sort of large-scale break. northeast of Chewelah and north of there on the west Although there are significant facies changes, fairly flank of Eagle Mountain. The actual fault plane or fault good stratigraphic correlations can be made between zone is not exposed on Embry Hill but can be located the Belt Supergroup in the Coeur d'Alene district, at precisely enough to confidently show that the fault dips Clark Fork, and in the report area. The facies changes shallowly to the west under the Deer Trail Group. The are not unreasonable, considering the distances in- 68 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

w ·E thrust from east to west, as no section resembling the Deer Trail Group is known east of Chewelah. For the same reason, if the Deer Trail Group is thrust over the Belt Series, as is suspected, the upper plate must be thrust from west to east. Unfortunately there are no exposures of the Deer Trail Group or Belt A Supergroup west of the relatively narrow magnesite belt that would allow an estimate of how much move­ ment has occurred. The facies changes in the Belt sec­ tion between Clark Fork and the report area are more pronounced than those between the report area and w the magnesite belt. This suggests that the amount of thrusting may not be too great. Unfortunately, a complete section of Addy Quartz­ ite resting on the Belt Supergroup is nowhere exposed in the area. Because systematic differences in thick­ ness of the basal Cambrian quartzite are known to B exist in the region, comparison of the thickness of the quartzite on the Belt Supergroup with that above the Deer Trail Group might furnish information on how far the two sections were originally separated. Addy w E Quartzite overlying the Belt rocks east of the area should be examined with this in mind. Paleozoic rocks on Addy Quartzite appear to be different above the two Precambrian sections, but the Paleozoic carbonate rocks above the Belt Supergroup c are not well enough nor extensively enough exposed to make meaningful comparisons with those above the EXPLANATION Deer Trail Group. In the Hunters quadrangle, which includes the southern part of the magnesite belt, Camp­ bell and__ Raup (1964) showed several thousand feet of Addy Quartzite Belt Supergroup Deer Trail Group limestone, slate, and chert of Ordovician age or older FIGURE 24.-Diagrammatic cross sections showing possible above the quartzite, where it overlies the Deer Trail interpretations of thrust relations between the Belt Super­ Group. Yates (1964) also showed several thousand group and Deer Trail Group. A, Simple thrust relation­ ship. Deer Trail Group thrust eastward over the Belt feet of carbonate, phyllite, and slate of Ordovician age Supergroup. This is considered the most likely relation or older above the quartzite, all of which apparently and is the one shown on the geologic map and sections. overlies the Deer Trail or Priest River Group, but no B, Belt Supergroup thrust westward over the Deer Trail rocks corresponding to either of these thick sections Group. Thrust later displaced by the Jumpoff Joe fault are found above the quartzite resting on the Belt Super­ and upper part removed by erosion. C, Deer Trail Group thrust eastward over the Belt Supergroup. Thrust is in group. Throughout the report area, Mississippian car­ subsurface only and is displaced by the Jumpo:ff Joe fault. bonate rocks are found above the Addy Quartzite where This interpretation assumes the thrusting is Precambrian it rests on the Belt rocks, but they have not been found in age. anywhere else in northeastern Washington. The J umpoff Joe fault is thus presumably younger than the Mississippian carbonate rocks and is probably volved. No large-scale thrust faults are known in the younger than the Starvation Flat Quartz Monzonite area between these localities, and so if the Belt section and the Phillips Lake Granodiorite. This would indicate of the report area had been thrust over the Deer Trail that the thrusting took place less than 100 m.y. ago. Group, the rest of the Belt rocks east to the Clark Fork Although it has been suggested here that the Jump­ and Coeur d'Alene areas would have had to move with off Joe fault is the major structure separating the Deer it. The thrust sheet would be tremendously large and Trail and Belt blocks, it is possible that the large north­ would contain no known imbricate thrusts, and the east-striking faults that pass a few miles northwest of thrust plane would be nowhere exposed at the surface. Chewelah mark the division between the blocks and If this is the case, however, the upper plate would be that one of the faults in that system is significantly MINERAL DEPOSITS 69 larger than the others. If this were the case, several Most of these faults are not exposed, but are inferred interpretations that have been proposed would have because of anomalous relationships across alluvium­ to be altered: (1) The rocks west of the Jumpoff Joe filled valleys. The faults immediately east and west of fault that have been assigned to the Deer Trail Group, the Jumpoff Joe fault appear to belong to this group. primarily on the basis of their association with the These faults may have been active at the same time as greenstone of the Huckleberry Formation, are actually the large northeast-striking faults, some of which are part of the Belt Supergroup, (2) the greenstone, al­ interpreted as cutting the north-south faults and some though thin, is present within the eastern structural as being cut by them. The fault separating the Addy block, and (3) the Jumpoff Joe fault, although a large Quartzite from the Paleozoic carbonate rocks southeast fault, has not had a significant amount of lateral move­ of Chewelah probably belongs to this group. ment along it. Two sets of northeast-striking faults are found in the There are two significant reasons for suggesting that Belt Supergroup block, but one set has a consistently this other fault sys~m, and not the J umpoff Joe fault, different strike (N. 50°-60°E.) from the faults in· the is the major structure separating the two structural Deer Trail Group block. The large fault that passes blocks: ( 1) All rocks and structures with the northeast through the Upper Cottonwood Road area is repre­ trend so consistently found in the Deer Trail block sentative of that group. The other set, which approxi­ would be restricted to that block. Most of the rocks in mates the N.30°-40° E. strike of the major faults in the report area west of the J umpoff Joe fault and south the Deer Trail Group block, is not represented in the of Chewelah do not have this northeast strike. To sug­ report area. Three large faults with this trend are gest stratigraphic assignments on the basis of struc­ found south of Horseshoe Lake, about 4 miles east of tural evidence in this manner is ordinarily not justified. Blue Grouse Mountain. However, the consistency of the northeast trends within West of Loon Lake, three shear zones are found in the Deer Trail block is so striking and is developed over the 50-m.y.-old Silver Point Quartz Monzonite. These such a large area that any rocks assigned to this block zones strike about north-south to north-northwest. not having the northeast trend should be regarded with Within the zones the rock is highly brecciated and has some suspicion. (2) The topographic lineament formed been recemented by chloritic material and quartz. by faults of this zone within the report area continues Aplite dikes in the zones are both twisted and broken, to the southwest outside the area. Campbell and Loof­ as if the process which formed the shear zones may have bourow (1962, pl. 1) mapped a fault along part of this been active when the dikes were intruded and con­ lineament, but the fault passes beyond the limits of tinued active for some time after the dikes solidified. their mapping a short distance to the south. Numerous smaller faults, some inferred, some ob­ Considering stratigraphic and structural relations served, are shown on the geologic maps. Most cannot jointly, the following correlations and sequence of be dated by crosscutting relationships with closely events are tentatively proposed to explain the relation dated rocks or other structures. between the Deer Trail Group and Belt Supergroup: (1) The Deer Trail Group is roughly equivalent to the MINERAL DEPOSITS upper Wallace Formation and the Striped Peak For­ mation; (2) the strata of the two blocks were deposited Commodities of economic interest in the report area at localities more widely separated than at present, include copper, silver, lead, tungsten, barite, clay, silica although information is not sufficient to accurately esti­ sand, and feldspar. Individual mine workings and pros­ mate how far apart they were; (3) the two groups pects that were accessible between 1963 and 1968 were underwent separate, though possibly related, deforma­ described in detail by Clark and Miller (1968, p. 4-5 tion at their respective sites of deposition; and (4) and pl. 2) and Miller ( 1969, p. 6). Clark and Miller also sometime after the extrusion of the Huckleberry vol­ described from old company mine maps some workings canics and deposition of the Addy Quartzite, and prob­ in the Eagle Mountain area that were not accessible ably after intrusion of the Starvation Flat Quartz Mon­ during the study period. Only general mine areas will zonite, the two sections were brought together by thrust be discussed here. The reader is referred to earlier faulting. reports by Weaver (1920), Patty (1921), Huntting OTHER FAULTS (1956), Clark and Miller (1968), and Miller (1969) for individual mine descriptions. Several of the faults that have a general north-south In terms of total value of ore produced, the copper­ strike appear to be older than the Flowery Trail Grano­ silver mines around Eagle Mountain are by far the diorite because the brecciated rocks in one of the fault most important in the area. Of the 10,551,098 pounds zones near the pluton are thoroughly recrystallized. of recorded copper production from the report area, 70 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

about 94 percent is accredited to the mines around molybdenite. The Embry Hill veins may have been Eagle Mountain (Fulkerson and Kingston, 1958, p. emplaced within the southern continuation of the Eagle 27-28). Almost 100 percent of the recorded silver pro­ Mountain shear zone or formed in subsidiary breaks duction also comes from these mines. related to the Jumpoff Joe fault; the latter may border The copper-silver ore is found in quartz-carbonate the west margin of the mineral deposits at this locality. veins which range in thickness from a few inches to over A few deposits of lesser importance are found on the 25 feet. Most are between 10-25 feet wide and are south side of the Flowery Trail Granodiorite. Only the within or near well-developed steeply dipping north­ Jay ·Gould mine has had recorded production. The northeast shear zones confined to the upper and lower veins in the Jay Gould area are typified by lead and parts of the Wallace Formation. Sulfides are sparsely silver--bearing minerals and thus differ from the Eagle disseminated in the veins and host rock and include Mountain and Embry Hill ores. Argentiferous galena, pyrite, chalcopyrite, tetrahedrite, pyrrhotite, covellite, the major ore mineral, is found in quartz veins as much and chalcocite or digenite. Malachite and hematite are as 10 feet wide. also present. Most of the ·mines in the north half of the The distribution of base metal deposits around the report area, including those around Eagle Mountain, Flowery Trail Granodiorite appears to exhibit a zona­ are in country rock peripheral to the Flowery Trail tion of sorts. The lead-silver deposits of the Jay Gould Granodiorite, and the mineralization may be related to area and the Blue Star mine of the Eagle Mountain that pluton. area occur within the margins of the pluton and in the The sections accompanying the preliminary geologic adjacent sedimentary rocks. Next and relatively near map by Clark and Miller (1968, pl. 1) show Eagle the contact are the silver-copper deposits of Embry Mountain to be underlain by a nearly horizontal thrust Hill, in which the silver-copper ratio is relatively low. fault. However, additional work since the publication of The Eagle Mountain deposits, which have a high silver­ that map and the delineation of individual Belt forma­ copper ratio, are farthest from the pluton. tions suggest that the mountain probably is not floored The only other base metal deposit in the report area by a thrust fault and that the shear zones which appear which has had any significant production is the Loon to control the location of the veins are probably con­ Lake copper mine, in .the NE* sec. 33, T. 31 N., R. 41 tinuous at depth. Both the thrust and nonthrust inter­ E. It has a production record of 622,555 pounds of cop­ pretations are based on poor exposure in critical areas, per and 532 ounces of silver (Fulkerson and Kingston, however, and should be reevaluated if additional data 1958, p. 45). The production was recorded principally become available. for the period between 1916 and 1919 and came from Only minor exploration has ever been attempted an individual ore shoot in a quartz vein from 4 to 20 below the main adit level in any of the mines in the feet wide. Most of the ore came from a secondary zone Eagle Mountain area. Regardless of whether the area of azurite, malachite, and cuprite. No new shoots were . is floored by a thrust fault or not, the vein system found by additional exploration. No other base metal should be explored at depth by drilling. Patty (1921, mines with any production are located within several fig. 8) showed a rich streak of tetrahedrite continuing miles of the Loon Lake copper mine, although numerous 350 feet below the main adit level in one mine and large quartz veins similar to the one on which the prop­ widening with depth. In addition, old company maps erty is developed are found in the mountains to the and records indicate an increasing silver-coppel' ratio east and southeast. at depth. The ever-increasing demand for silver pos­ Several tungsten deposits have been found on Blue sibly warrants exploratory drilling of the lower parts of Grouse Mountain east of Deer Lake. They consist of the vein system. sparsely disseminated huebnerite crystals in quartz Smaller deposits have been mined on Embry Hill, veins, greisen, and pegmatite segregations around the about 1 mile southwest of the Eagle Mountain area. periphery of the muscovite quartz monzonite. The min­ These are also on the north side of the Flowery Trail eralization appears to be related to this quartz monzo­ Granodiorite. They are roughly on strike with the Eagle nite pluton. Mountain vein system, but whether the two are con­ Barite deposits have been found on Eagle Mountain, tinuous is uncertain because faults and an area of poor the hills north and east of Valley, and a few hundred exposure separate the two localities. Only 4,011 pounds feet southwest of the Loon Lake copper mine. Moen of copper and 22 ounces of silver have been recovered (1964) described all the deposits, gave production from the ore of the Embry Hill mines (Fulkerson and figures, and estimated reserves. All the barite occurs Kingston, 1958, p. 28). The deposits differ from those in veins or a series of veins which range in width from on Eagle Mountain in that they have a much higher less than 1 inch to several tens of feet. Moen reported copper-silver ratio and also contain small amounts of 634 tons of barite shipped, reserves of 2,300 tons meas- REFERENCES CITED 71

ured and indicated, and 24,750 tons inferred. The veins area, Washington: Harvard Univ., Cambridge, Ph.D. are scattered and appear to be unrelated, but all are thesis, 149 p. Branson, C. C., 1931, New paleontological evidence on the age restricted to the Striped Peak Formation or its probable of the metamorphic series of northeastern Washington: equivalent, the Deer Trail Group. Science, v. 74, no. 1907, p. 70. Two claypits in the Latah Formation are located Campbell, A. B., 1960, Geology and mineral deposits of the St. southeast of Deer Lake, near the south edge of the map Regis-Superior area, Mineral County, Montana: U.S. Geol. area. Similar deposits are found less than 1 mile east of Survey Bull. 1082-1, p. 545-612 [1961]. Campbell, A. B., and Good, S. E., 1963, Geology and mineral the report area at the same latitude as those southeast deposits of the Twi:ri Crags quadrangle, Idaho: U.S. Geol. of Deer Lake and just south of the area at the town of Survey Bull.1142-A, p. A1-A33. Clayton. Hosterman (1969, p. 55, 56) studied the clay Campbell, A. B., and Raup, 0. B., 1964, Preliminary geologic deposits of Spokane County in detail. He described map of the Hunters quadrangle, Stevens and Ferry the deposit just east of the area and gave analyses of Counties, Washington: U.S. Geol. Survey Mineral Inv. the clay. Miller (1969, pl. 1) prepared a highly general­ Field Studies Map MF-276, scale 1:48,000. Campbell, Ian, and Loofbourow, J. S., 1962, Geology of the ized map which shows the possible location of addi­ magnesite belt of Stevens County, Washington: U.S. Geol. tional clay in nearby areas. Survey Bull. 1142-F, p. F1-F53. The Addy Quartzite may in part be pure enough to Carlisle, D. 1963, Pillow breccias and their aquagene tuffs, be a source for silica sand. Above the purple and pink Quadra Island; British Columbia: Jour. Geology, v. 71, zones at the base of the formation, the sand is generally no. 1, p:48-71. · Chayes, Felix, 1956, Petrographic modal analysis-an elemen­ white or light gray and probably almost pure quartz. tary statistical appraisal: New York, John Wiley & Sons, An extensive sampling program would be necessary to 113p. block out an area of sufficient purity. On Lane Moun­ Clark, L. D., and Miller, F. K., 1968, Geology of the Chewelah tain, a few miles west of the area, the quartzite is being Mountain quadrangle, Stevens County, Washington: mined for the manufacture of glass. There, however, Washington Div. Mines and Geology Map. GM-5, scale 1:62,500. the silica cement has been removed or was never depos­ Daly, R. A., 1912, Geology of the North American Cordillera ited, and the rock is extremely friable. All the Addy at the forty-ninth parallel: Canada Geol. Survey Mem. 38, Quartzite in the report area would require considerable 1-3) 857 p. crushing. Dings, M. G., and Whitebread, D. H., 1965, Geology and ore Feldspar for use as a flux in glass manufacture may deposits of the Metaline ziRc-lead district, Pend Oreille County, Washington: U.S. Geol. Survey Prof. Paper 489, be available from the ·muscovite quartz monzonit~. 109 p. Analyses of this unit (see section "Muscovite Quartz Doe, B. R., and Hart, S. R., 1963, The effect of contact meta­ Monzonite, Petrology"). show that it contains very morphism on lead in potassium feldspars near the Eldora little iron or manganese, two of the chief contaminants Stock, Colorado: Jour. Geophys. Research, v. 68, no. 11, p. in glass making. Abundant, easy-to-reach reserves are 3521-3530. Embrysk, B. J., 1954, Additions to the Devonian and Mississip­ available in the southeastern part of the area and just pian paleontology of northeastern Washington: Washing­ east of the area. ton State Coli., Pullman, Master's thesis. Eskola, Pentti, 1952, On the granulites of Lapland: Am. Jour. REFERENCES CITED Sci., Bowen volume, pt. 1, p. 133-171. Fulkerson, F. B., and Kingston, G. A., 1958, Mine production of gold, silver, copper, lead, and zinc in Pend Oreille and Anderson, A. L., 1940, Geology and metalliferous deposits of Stevens Counties, Washington, 1902-1956: U.S. Bur. Mines Kootenai County, Idaho: Idaho Bur. Mines and Geology Inf. Circ. 7872, 45 p. Pamph. 53, 67 p. Barnes, C. W., 1965 Reconnaissance geology of the Priest River Fyfe, W. S., Turner, F. J., and Verhoogen, John, 1958, Meta­ area, Idaho: Wisconsin Univ., Madison, Ph.D. thesis,145 p. morphic reactions and metamorphic facies: Geol. Soc. Bateman, P. C., Clark, L. D., Huber, N. K., Moore, J. G. and America Mem. 73, 259 p. Rinehart, C. D., 1963, The Sierra Nevada batholith-a Glover, S. L., 1941, Clays and shales of Washington: Washing­ synthesis of recent work across the central part: U.S. Geol. ton Div. Mines and Geology Bull. 24, 368 p. Survey Prof. Paper 414-D, p. D1-D46. Griggs, A. B., 1966, Reconnaissance geologic map of the west Becraft, G. E., 1966, Geologic map of the Wilmont Creek quad­ half of the Spokane quadrangle, Washington and Idaho: range, Ferry and Stevens Counties, Washington: U.S. Geol. U.S. Geol. Survey Misc. Geol. lnv. Map 1-464, scale 1: Survey Geol. Quad. Map GQ-538, scale 1:62,500. 125,000. Becraft, G. E., and Weis, P. L., 1963, Geology and mineral Hanson, G. N., and Gost, P. W., 1967, Kinetic studies in con- L­ deposits of the Turtle Lake quadrangle, Washington: U.S. tact metamorphism zones: Geochim. et Cosmochim. Acta, Geol. Survey Bull. 1131, 73 p. V. 31, p. 1119-1153. Bennett, W. A. G., 1941, Preliminary report on the magnesite Harrison, J. E., and Campbell, A. B., 1963, Correlations and deposits of Stevens County, Washington: Washington Div. problems in Belt Series stratigraphy, northern Idaho and Geology Rept. lnv. 5, 25 p. western Montana: Geol. Soc. America Bull., v. 74, no. 12, Bowman, E. C., 1950, Stratigraphy and structure of the Orient p. 1413-1428. 72 CHEWELAH-LOON LAKE AREA, STEVENS AND SPOKANE COUNTIES, WASHINGTON

Harrison, J. E., and Jobin, D. A., 1963, Geology of the Clark Ransome, F. L., 1905, Ore deposits of the Coeur d'Alene dis­ Fork quadrangle, Idaho-Montana: U.S. Geol. Survey Bull. trict, Idaho: U.S. Geol. Survey Bull. 260, p. 274-303. 1141-K, 38 p. Ransome, F. L., and Calkins,· F. C., 1908, The geology and ore Hart, S. R., 1961, Mineral ages and metamorphism, in Kulp, deposits of the Coeur d'Alene district, Idaho: U.S. Geol. J. L., ed., Geochronology of rock systems: New York Acad. Survey Prof. Paper 62, 203 p. Sci. Annals, v. 91, art. 2, p. 192-197. Reynolds, R. L., 1968, Paleocurrents, petrology, and source area ---1964, The petrology and isotopic-mineral age relations of the Addy Quartzite (Cambrian), northeastern Washing­ of a contact zone in the Front Range, Colorado: Jour.· ton: Undergraduate thesis, Princeton Univ., 48 p. Geology, v. 72, no. 5, p. 493-525. Rice, H. M.A., 1941, Nelson map-area, east half, British Colum­ Hobbs, S. W., Griggs, A. B., Wallace, R. E., and Campbell, bia: Canada Geol. Survey Mem. 228, Pub. 2460,86 p. A. B., 1965, Geology of the Coeur d'Alene district, Sho­ Rinehart, C. D., and Fox, K. F., 1972, Geology and mineral shone County, Idaho: U.S. Geol. Survey Prof. Paper 478, deposits of the Loomis quadrangle, Okanogan County 139 p. Washington: Washington Div. Mines and Geology Bull. Hosterman, J. W., 1969, Clay deposits of Spokane County, no. 64, 124 p. Washington: U.S. Geol. Survey Bull. 1270, 96 p. Ross, C. P., 1963, The Belt series in Montana: U.S. Geol. Sur­ Huntting, M. T., 1956, Inventory of Washington minerals; pt. vey Prof. Paper 346, 122 p. [1964]. 2, Metallic minerals: Washington Div. Mines and Geology Schroeder, M. C., 1952, Geology of the Bead Lake district, Pend Bull. 37, v. 1, 428; p. v. 2, 67 p. Oreille County, Washington: Washington Div. of Mines Huntting, M. T., Bennett, W. A. G., Livingston, V. E., Jr., and and Geology Bull. 40, 57 p. Moen, W. S., 1961, Geological map of Washington: Olym­ Shenon, P. J., and McConnel, R. H., 1939, The Silver Belt of pia, Washington Div. Mines and Geology, scale 1: 500,000. the Coeur d'Alene district, Idaho: Idaho Bur. Mines and Jones, R. H. B., 1928, Notes on the geology of the Chewelah Geology Pamph. 50, 8 p. quadrangle, Stevens County, Washington: Northwest Sci., Shido, F., 1958, Plutonic and metamorphic rocks of the Nakoso v. 2, p. 111-116. and Iritono districts in the central Abukuma Plateau: ---1929, The geology and mineral resources of the Chewe­ Tokyo Univ., Jour. Faculty Sci., sec. 2, v. 11, p. 171. lah quadrangle, Washington: Washington State Univ., Shido, F., and Miyashiro, A., 1959, Hornblendes of basic meta­ Seattle, Master's thesis, ()9 p. morphic rocks: Tokyo Univ., Jour. Faculty Sci., sec. 2, Kirkham, V. R. D., and Johnson, M. M., 1929, The Latah for­ v.12, p. 86. mation in Idaho: Jour. Geology, v. 37, no. 5, p. 483-501. Turner, F. J., and Verhoogen, John, 1960, Igneous and meta­ Klapper, G., 1966, Upper Devonian and Lower Mississippian morphic petrology [2d ed.]: New York, McGraw-Hill, conodont zones in Montana, Wyoming, and South Dakota: 694 p. Kansas Univ. Paleo. Contr. 3, 43 p. Tuttle, 0. F., and Bowen, N. L., 1958, Origin of granite in the

Laniz,-R. V., Stevens, R. E., and Norman, M. B., 1964, Staining light of experimental studies in the system NaA1Si30 8- of plagioclase feldspar and other minerals with F. D. and Si02-H20: Geol. Soc. America Mem. 74, 153 p. C. Red No. 2, in Geological Survey research 1964: U.S. Walker, J. F., 1926, Geology and mineral deposits of Winder­ Geol. Survey Prof. Paper 501-B, p. B-152-B153. mere map area, British Columbia: Canada Geol. Survey Miller, F. K., 1969, Preliminary geologic map of the Loon Lake Mem. 148, 69 p. quadrangle, Stevens and Spokane Counties, Washington: Wallace~ R. E., and Hosterman, J. W., 1956, Reconnaissance Washington Div. Mines and Geology Geol. Map GM-6, geology of western Mineral County, Montana: U.S. Geol. scale 1: 62,500. Survey Bull. 1027-M, p. 575-612. Mills, J. W. and Yates, R. G., 1962, High-calcium limestones Wanless, R. K., Stevens, R. D., Lachance, G. R., and Rimsaite, of eastern Washington: Washington Div. Mines and Geol­ R. Y. H., 1965, Age determinations and geological studies: ogy Bull. 48, 268 p. Canada Geol. Survey Paper 64-17 (pt. 1), 126 p. Moen, W. S., 1964, Barite in Washington: Washington Div. Weaver, C. E., 1920, The mineral resources of Stevens County: Mines and Geology Bull. 51, 112 p. Washington Geol. Survey Bull. 20, 350 p. Nockolds, S. R., 1954, Average chemical composition of some Weis, P. L., 1968, Geologic map of the Greenacres quadrangle, igneous rocks: Geol. Soc. America Bull., v. 65, no. 10, p. Washington and Idaho: U.S. Geol. Survey Geol. Quad. 1007-1032. Map GQ-734, scale 1: 62,500. Okulitch, V. J., 1951, A lower Cambrian fossil locality near Yates, R. G., 1964, Geologic map and sections of·the Deep Addy, Washington: Jour. Paleontology, v. 25, no. 3, p. 405- Creek area, Stevens and Pend Oreille Counties, Washing­ 407. ton: U.S. Geol. Survey Misc. Geol. lnv. Map 1-412, scale Pardee, J. T., and Bryan, Kirk, 1926, Geology of the Latah 1:31,680. formation in relation to the lavas of the Columbia Plateau Yates, R. G., Becraft, G. E., Campbell, A. B., and Pearson, near Spokane, Washington: U.S. Geol. Survey Prof. Paper R. C., 1966, Tectonic framework of northeastern Washing­ 140--A, p. 1-16. ton, northern Idaho, and northwestern Montana: Canadian Park, C. F., Jr., and Cannon, R. S., Jr., 1943, Geology and ore lnst. Mining and Metallurgy Spec. Vol. 8, p. 47-59. deposits of the Metaline quadrangle, Washington: U.S. Yates, R. G., and Engels, J. C., 1968, Potassium-argon ages of Geol. Survey Prof. Paper 202, 81 p. some igneous rocks in northern Stevens County, Washing­ Patty, E. N., 1921, The metal mines of Washington: Washing­ ton, in Geological Survey research 1968: U.S. Geol. Survey ton Geol. Survey Bull. 23, 366 p. Prof. Paper 600--D, p. D242-D247. INDEX

[Italic page numbers indicate major references]

Page Page Page A Cliff Ridge ...... 26, 61 Fossils-Continued Cliff Ridge Lookout ...... SS Composita-Continued Accessibility ...... Coeur d'Alene district...... 4, 6, 7, 10, 11, 12, sp ...... 32 Addy Quartzite ...... 2, B7 2S, 66, 67 Con~onts ...... 82 faults ·····································:·················· 64, 65 Columbia River Group ...... 4, 61 Corals ...... 81 folds...... 66 Colville-Loon Lake batholith...... SS, 51 Crinoids ...... 88 mineralization ...... 71 Colville River ...... 62 Crurith.yris sp ...... 82 Cyrtospirif13r sp ...... 8S relation to Belt Supergroup...... 24, 68, 69 Colville Valley ... ~ ...... 62, 66 relation to Deer Trail Group ...... 68, 69 Conglomerate ...... 2, 24, 25, 26, 61 Cystodictya sp ...... 8S relation to glacial deposits...... 62 Copper ...... 69, 70 Endothyra sp ...... 83 relation to granodiorite...... 50 Cottonwood Creek ...... 1S, SO, 58, 65 Fenestella sp ...... 83 relation to Huckleberry Formation.... 24 Crossbedding ...... 7, 10 Fucoids ...... 28 relation to Striped Peak Formation.. 11, 16 Cross-laminations ...... 6, 7, 8, 10, 11, 14, 16 Gastropods ...... 82 Ahren Meadows ...... 47 Crowell-Sullivan Ridge ...... 29 Globovalvulina buUoides ...... S3 Allanite ...... 49, 51 Crystallization ...... 41, 45, 48 Gnathodus sp ...... 82 Alluvial deposits ...... 61 Graphiodactylus tenuis ...... S2 Hyolithes sp ...... 28 Amphibolite ...... 24 D Andesite ...... 60 HyolitheUus sp ...... 28 J onesina craterigera ...... S2 Apatite ...... 4S, 44, 47, 49, 50, 51 Dating methods ...... 52 Kutorgina ...... 28 Aplite ...... 4, 4S, 50, 69 Deep Creek ...... 29, 55 cingulata ...... 28, S2 Argillite ...... 2, 4, 6, 7, 8, 12, 1S, 16, Deer Lake ...... 4, 6, 7, 45, 56, 62, 64, 70 sp ...... 28 17,1~19,2~2~66 Deer Lake Mountain ...... 8, 10, 11, 12, 28, 45, 65 Leptargonia analoga ...... 83 Armstrong, A. K., quoted ...... SO, S2 Deer Mountain ...... 66 Linnarsso-nia ...... SO Deer Park ...... 46 B LophophyUidium proliferum ...... S8 Deer Trail Group ...... 2, 16, 62 Micromitra (Paterina) sp ...... 28 relation to Belt Supergroup ...... 18 Bald Mountain ...... 8, 10 MillereUa sp ...... 33 structure ...... 65 Barite ...... 69, 70 Nevadella addyensis ...... 28 undivided ...... 18 Basalt ...... :...... 60 Nevadia [=NevadeUa] addyensus...... 28 Deformation ...... ,. 11, 14 Batholiths ...... 4, 16, SS, 51, 55 Olenellids ...... 28 Dikes ...... 8, 24, S6, SS, 40, 41, 50, 51, 57, 58 Bayley Creek ...... 26, 67 Olenoides ...... 80 Dolomite ...... 13, 17, 26, 30, 31, 32 Bend Lake ...... 2, 18, 6S, 67 Ostracodes ...... 82 Duncan, Helen, quoted...... 33 Bear Canyon ...... 42, 67 Pelmatozoan debris ...... SO, S1, S2 Dutro, J. T., quoted...... 33 Bear Creek ...... 62 Peronopsis ...... 80 Beitey Lake ...... 12, 15, 16 Platyceras sp ...... S2 Bell Meadow ...... 42 E Pseudopolygnathus multistriata ...... 32 Belt Supergroup ...... 2, -'• 65 Pseudosyrinz sp ...... S2 dikes ...... 58 Eagle Mountain...... 8, Rhabdammina sp ...... 88 relation to Deer Trail Group...... 18 11, 12, 14, 15, 27, 29, so. 36, 42, 67, 69, 70 Rhipidomella sp ...... S2 Edna Dolomite ...... 17, 66 structure ...... 6B Rhombopora nitidula ...... SS Benson Peak ...... 46 relation to Striped Peak Formation.... 2S RusteUa edso-ni ...... 28 Blacktnll Mountain ...... SS, 56 Embry Hill ...... 67, 70 Siphondella isosticha ...... S2 Blue Grouse Mountain ...... 8, 11, 46, 64, 69, 70 Embry Hill mines...... 70 Spirifer rockymo-ntanus ...... S3 Blue Star mine ...... S6, 70 sp ...... 82 Branson, C. C., quoted...... S2 F Squamularia transversa ...... 3S Breccia ...... 17, 24 Stromatolites ...... 18 Brecciated rocks ...... 69 Fan Lake...... 62 Syringothyris sp ...... S3 Brecciation ...... 42 Faults ...... 4, 8, 38, 64, 67, 69 Tenticospirifer utahensis ...... SS Brewer Mountain ...... :...... 42 Feldspar ...... 69, 71 Tetracamera sp ...... 38 Buffalo Hump Formation...... 18 Flowery Trail Granodiorite...... 3S, 52, 57 Trilobites ...... 28, 80 relation to Belt Supergroup...... 24 relation to mineralization...... 70 Trochammina sp ...... 33 Burke Formation ...... 6 relation to Phillips Lake Unispirifer sp ...... 32 relation to Silver Point Quartz Granodiorite ...... 4S Monzonite ...... 49 Flows ...... 24 G Burnt Valley ...... 61 Folds ...... ,...... 4, 6B Fossils: Garnet ...... 48, 47 c Acrothele ...... SO Geologic setting ...... 4 A mmobaculites sp ...... SS Gerome Andesite ...... 60 Callspell Peak ...... SS, 40, 41, 4S, 62 Amphissites centro-notus ...... S3 Glacial deposits ...... 61 Camas Basalt ...... 61 simplicissimus ...... S3 Goddards Peak ...... 6, 24, 86, 40, 42, 62 Carbonate rock ...... 8, B9, 90, 91, as Ample:x:izaphrentis sp ...... 31 Chewelah Argillite ...... :...... 24 Archaeocyatha ...... 32 Gold Hill ...... ,...... 66 Chewelah Creek ...... 67 Bathyuriscus ...... SO Graded bedding ...... 6, 11, 14, 16 Chewelah Mountain ...... 5, 6, 62, 64 Brachiopods ...... 30, S2, 33 Granodiorite ...... 86, 40, 50, 56 Chewelah syncline ...... 6S, 64, 65 Bryozoans ...... ,...... S1 relation to Silver Point Quartz Clark Fork ...... 67 CaveUina coreUi ...... :...... 32 Monzonite ...... 57 Clay ...... 61, 69, 71 Composita ...... ,...... :...... 33 Greenstone .:;...... 24 73 74 INDEX

Page Page Page Grouse Creek ...... 7, 10, 62 Mud-chip breccias ...... 7, 8, 12 Shale ...... 28 Gypsy Creek ...... 26 Mud cracks ...... 6, 7, 8, 10, 11, 12, 14, 16 Shearing ...... 42 Gypsy Quartzite ...... 27, 29 Shedroof Conglomerate ...... 25 N,O Sherwood Creek ...... 62 H Silica ...... 69, 71 Nelson Peak ...... 5, 6, 24, 44 Sills ...... 6, 8, 24, 36, 41 Hope fault ...... 65 Nelson Peak anticline...... 63, 64, 65 Siltite ...... 4, 6, 7, 8, 12, 13, 16, 18, 19 Horseshoe Lake ...... 4, 62, 69 Newport Group ...... , ...... 19, 67 Siltstone ...... 61 Huckleberry Formation ...... 2, 24, 66 Newport syncline ...... ,...... 63, 64 Silver ...... 69, 70 relation to Starvation Flat Quartz Silver Point Quartz Monzonite...... 47, 52, 66, 57 Monzonite ...... 38 Old Dominion Limestone...... 29,32 dikes ...... 68, 59 Huckleberry Mountain ...... 16, 40 Osburn fault ...... 65 faults...... 69 Huckleberry volcanics ...... 69 relation to dikes...... 57 Hydrothermal alteration ...... 8 p ··relation ·to granodiorite...... 57 Hypabyssal rocks ...... 58 relation to quartz monzonite, fine- Palmer, A. R., quoted...... 30 grained ...... 57 I, J, K Parker Mountain ...... 10, 13 Slate ...... 17,26 Peewee syncline ...... 63 Snow Valley anticline...... 63, 64 Ilmenite 37 Pegmatite ...... 41, 43 South Fork of Chewelah Creek...... 6, 62 Irene Conglomerate ...... 25 Pend Oreille Lake...... 6, 10, 63 Sphene ...... :'-..49,~50, r51 Irene Volcanics ...... 25, 66 Pend Oreille V.:alley...... 24 Spirit pluton ...... 55 Iron ...... 25, 61 Phillips Lake ...... 59 Starvation Flat Quartz Iron Mountains ...... 65 Phillips Lake Granodiorite...... 40, 55, 57 Monzonite ...... 98, 51, 52, 53, 57 dikes ...... 58 relation to Belt Supergroup and Jay Gould mine...... 36, 37, 52, 70 relation to Flowery Trail Deer Trail Group...... 69 Jay Gould Ridge ...... 7, 58, 64 Granodiorite ...... 36 relation to Priest Lake Group...... 67 Johnson Mountain...... 64 relation to Starvation Flat Quartz Starvation Lake ...... 39 Joints .... ."...... 42 Monzonite ...... 38, 67 Stensgar Dolomite ...... 17, 66 Jumpoff Joe fault...... 62, 66, 67, 69, 70 Plutonic rocks ...... 4, 99, 47, 51, 52 relation to andesite...... 60 Jumpoff Joe Lake...... 18, 25, 29, 67 Previous work ...... B relation to Striped Peak Jumpoff Joe Mountain...... 11, 12, 27, 45, 60 Prichard Formation ...... 3, 4 Formation ...... 23 Juno Echo mine...... 36 relation to Togo Formation...... 19 Stratigraphic correlations ...... 67 Prichard Lake ...... 4 Striped Peak Formation...... 11 Kaniksu batholith ...... 33, 51, 55 Priest Lake ...... 63 relation to Belt Supergroup...... 19 Klapper, Gilbert, quoted...... 32 Priest River Group ...... 16, 25, 66, 67 relation to Deer Trail Group...... 69 Priest River syncline...... 63, 64 relation to glacial deposits...... 62 L Purcell Series ...... 4, 66 relation to Togo Formation...... 23 Pyrite ...... 47 Structure ...... 4, 6£ Lane Mountain ...... 71 Latah Formation ...... 61 Q,R T Lead ...... 69, 70 Lenhart Meadows ...... 40 Quartz monzonite ...... 39,47 Tacoma Creek ...... 40 Leola Volcanics ...... 25 coarse-grained ...... 45, 56, 57 Talus deposits ...... 61 Libby Formation ...... 24 fine-grained ...... 50, 56 Ten Mile Creek...... :...... 64 Limestone ...... 31, 32 relation to Silver Point Quartz The Tinderbox ...... 42, 67 Limonite ...... 47 Monzonite ...... 57 Thomason Creek ...... 62 Little Bear Creek...... 38 muscovite ...... ,46, 56, 57 Thrusts ...... 65, 67, 68, 70 Loon Lake ...... 29, 45, 47, 48, 56, 57, 69 two-mica ...... 44, 56 Toby Conglomerate ...... 66 Loon Lake copper mine...... 12, 15, 16, 70 Quartzite ...... 2, Togo Formation ...... 17, 66 Loon Lake Mountain ...... 8 4, 6, 7, 8, 19, 23, 24, 26, 27 relation to Prichard Formation...... 19 relation to Silver Point Quartz Quartzite Mountain...... 8, 10, 11, 12, 14, relation to Striped Peak Formation.... 23 Monzonite ...... 49 15, 27, 28, 36, 61, 62, 64, 65 Tourmaline ...... 43 Lost Creek ...... 38 Tuff ...... 24 Rattlesnake Hills ...... 44 Tungsten 69, 70 ·M Ravalli Group ...... 3, 6 Recrystallization ...... :...... 6, V,W,Y,Z McDonald Mountain ...... 8, 42 36, 37, 43, 44, 46, 48, 69 McHale Slate ...... 17, 66 Revett Formation ...... 7, 49 Volcanic rocks ...... 58 . relation to Striped .Peak Formation.... 23 .Ripple marks ...... 6, 7, 8, 10, 11, 12, 14, 16 Magnesite belt ...... 2, 16, 17, 18, 24, 62, 65 Wallace Formation ...... 8, 69, 70 Magnetite ...... 7, 37, 43, 47, 49, 50, 51 s relation to glacial deposits...... 62 Maitlen Phyllite ...... 29 relation to Silver Point Quartz Merriam, C. W., quoted...... 33 St. Regis Formation...... : 7 Monzonite ...... 49 Metaline Formation ...... 29 relation to Burke Formation...... 7 relation to Togo Formation...... 19 Metamorphic rocks ...... £4 relation to Silver Point Quartz Wilson Mountain...... 7, 42 Metamorphism ...... 6, 37, 45, 66 Monzonite ...... 49 Windermere Group ...... 14 Mineral deposits ...... 69 Salt casts ...... , ...... 12, 16 Windermere rocks ...... 4 Missoula Group ...... 11 Sand ...... 69 relation to Buffalo Hump .Formation.. 24 Sandstone ...... 2, 61 Yakima Basalt ...... 61 Monk Formation ...... 26, 66 Schist ...... 24 relation to Starvation Flat Quartz Scour and fill features ...... 16 Zircon ...... 7, 37, 43, 44, 47, 49, 50, 51 ··· ·Monzonite ...... 38· Sedimentary rocks ...... 58 Zoning ...... · 84